Guarded cover sheet for LCD polarizers and method of making the same

ABSTRACT

The invention generally relates to polymer films used as protective cover sheets for polarizer plates, their manufacture, and to a method for producing polarizing plates employing such polymer films. More particularly, the invention provides a guarded cover sheet composite comprising a temporary carrier substrate having a first cover sheet comprising a first low birefringence film on one side and a second cover sheet comprising a second low birefringence film on the other side.

FIELD OF THE INVENTION

The invention generally relates to polymer films used as protectivecover sheets for polarizer plates, their manufacture, and to a methodfor producing polarizing plates employing such polymer films. Moreparticularly, the invention provides a guarded cover sheet compositecomprising a temporary carrier substrate having a first cover sheetcomprising a first low birefringence film on one side and a second coversheet comprising a second low birefringence film on the other side.

BACKGROUND OF THE INVENTION

Transparent resin films are used in a variety of optical applications.In particular, resin films are used as protective cover sheets for lightpolarizers in variety of electronic displays, particularly LiquidCrystal Displays (LCD).

LCDs contain a number of optical elements that may be formed from resinfilms. The structure of reflective LCD's may include a liquid crystalcell, one or more polarizer plates, and one or more light managementfilms. Liquid crystal cells are formed by dispersing liquid crystalssuch as twisted nematic (TN) or super twisted nematic (STN) materialsbetween two electrode substrates. Polarizer plates are typically amulti-layer element of resin films and are comprised of a polarizingfilm sandwiched between two protective cover sheets. Polarizing filmsare normally prepared from a transparent and highly uniform amorphousresin film that is subsequently stretched to orient the polymermolecules and stained with a dye to produce a dichroic film. An exampleof a suitable resin for the formation of polarizer films is fullyhydrolyzed polyvinyl alcohol (PVA). Because the stretched PVA films usedto form polarizers are very fragile and dimensionally unstable,protective cover sheets are normally laminated to both sides of the PVAfilm to offer both support and abrasion resistance. Protective coversheets of polarizer plates are required to have high uniformity, gooddimensional and chemical stability, and high transparency. Originally,protective coversheets were formed from glass, but a number of resinfilms are now used to produce lightweight and flexible polarizers.Although many resins have been suggested for use in protective coversheets including, cellulosics, acrylics, cyclic olefin polymers,polycarbonates, and sulfones, acetyl cellulose polymers are mostcommonly used in protective cover sheets for polarizer plates. Polymersof the acetyl cellulose type are commercially available in a variety ofmolecular weights as well as the degree of acyl substitution of thehydroxyl groups on the cellulose backbone. Of these, the fullysubstituted polymer, triacetyl cellulose (TAC) is commonly used tomanufacture resin films for use in protective cover sheets for polarizerplates.

The cover sheet normally requires a surface treatment to insure goodadhesion to the PVA dichroic film. When TAC is used as the protectivecover film of a polarizer plate, the TAC film is subjected to treatmentin an alkali bath to saponify the TAC surface to provide suitableadhesion to the PVA dichroic film. The alkali treatment uses an aqueoussolution containing a hydroxide of an alkali metal, such as sodiumhydroxide or potassium hydroxide. After alkali treatment, the celluloseacetate film is typically washed with weak acid solution followed byrinsing with water and drying. This saponification process is both messyand time consuming. U.S. Pat. No. 2,362,580 describes a laminarstructure wherein two cellulose ester films each having a surface layercontaining cellulose nitrate and a modified PVA is adhered to both sidesof a PVA film. JP 06094915A discloses a protective film for polarizerplates wherein the protective film has a hydrophilic layer whichprovides adhesion to PVA film.

Some LCD devices may contain a protective cover sheet that also servesas a compensation film to improve the viewing angle of an imageCompensation films (i.e. retardation films or phase difference films)are normally prepared from amorphous films that have a controlled levelof birefringence either by uniaxial stretching or by coating withdiscotic dyes. Suitable resins suggested for formation of compensationfilms by stretching include polyvinyl alcohols, polycarbonates andsulfones. Compensation films prepared by treatment with dyes normallyrequire highly transparent films having low birefringence such as TACand cyclic olefin polymers.

Protective cover sheets may require the application of other functionallayers (herein also referred to as auxiliary layers) such as anantiglare layer, antireflection layer, anti-smudge layer, or antistaticlayer. Generally, these functional layers are applied in a process stepthat is separate from the manufacture of the resin film.

In general, resin films are prepared either by melt extrusion methods orby casting methods. Melt extrusion methods involve heating the resinuntil molten (approximate viscosity on the order of 100,000 cp), andthen applying the hot molten polymer to a highly polished metal band ordrum with an extrusion die, cooling the film, and finally peeling thefilm from the metal support. For many reasons, however, films preparedby melt extrusion are generally not suitable for optical applications.Principal among these is the fact that melt extruded films exhibit ahigh degree of optical birefringence. In the case of highly substitutedcellulose acetate, there is the additional problem of melting thepolymer. Cellulose triacetate has a very high melting temperature of270-300° C., and this is above the temperature where decompositionbegins. Films have been formed by melt extrusion at lower temperaturesby compounding cellulose acetate with various plasticizers as taught inU.S. Pat. No. 5,219,510 to Machell. However, the polymers described inU.S. Pat. No. 5,219,510 to Machell are not the fully substitutedcellulose triacetate, but rather have a lesser degree of alkylsubstitution or have proprionate groups in place of acetate groups. Evenso, melt extruded films of cellulose acetate are known to exhibit poorflatness as noted in U.S. Pat. No. 5,753,140 to Shigenmura. For thesereasons, melt extrusion methods are generally not practical forfabricating many resin films including cellulose triacetate films usedto prepare protective covers and substrates in electronic displays.Rather, casting methods are generally used to manufacture these films.

Resin films for optical applications are manufactured almost exclusivelyby casting methods. Casting methods involve first dissolving the polymerin an appropriate solvent to form a dope having a high viscosity on theorder of 50,000 cp, and then applying the viscous dope to a continuoushighly polished metal band or drum through an extrusion die, partiallydrying the wet film, peeling the partially dried film from the metalsupport, and conveying the partially dried film through an oven to morecompletely remove solvent from the film. Cast films typically have afinal dry thickness in the range of 40-200 microns. In general, thinfilms of less than 40 microns are very difficult to produce by castingmethods due to the fragility of wet film during the peeling and dryingprocesses. Films having a thickness of greater than 200 microns are also10 problematic to manufacture due to difficulties associated with theremoval of solvent in the final drying step. Although the dissolutionand drying steps of the casting method add complexity and expense, castfilms generally have better optical properties when compared to filmsprepared by melt extrusion methods, and problems associated withdecomposition at high temperature are avoided.

Examples of optical films prepared by casting methods include: 1.)Cellulose acetate sheets used to prepare light polarizers as disclosedin U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289 to Caelas well as more recent disclosures in U.S. patent application Ser. No.2001/0039319 Al to Harita and U.S. patent application Ser. No.2002/001700 A1 to Sanefuji; 2.) Cellulose triacetate sheets used forprotective covers for light polarizers as disclosed in U.S. Pat. No.5,695,694 to Iwata; 3.) Polycarbonate sheets used for protective coversfor light polarizers or for retardation plates as disclosed in U.S. Pat.No. 5,818,559 to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180 bothto Taketani; and 4.) Polyethersulfone sheets used for protective coversfor light polarizers or for retardation plates as disclosed in U.S. Pat.Nos. 5,759,449 and 5,958,305 both to Shiro.

Despite the wide use of the casting method to manufacture optical films,there are however, a number of disadvantages to casting technology. Onedisadvantage is that cast films have significant optical birefringence.Although films prepared by casting methods have lower birefringence whencompared to films prepared by melt extrusion methods, birefringenceremains objectionably high. For example, cellulose triacetate filmsprepared by casting methods exhibit in-plane retardation of 7 nanometers(nm) for light in the visible spectrum as disclosed in U.S. Pat. No.5,695,694 to Iwata. Polycarbonate films prepared by casting methodsexhibit in-plane retardation of 17 nm as disclosed in U.S. Pat. Nos.5,478,518 and 5,561,180 both to Taketani. U.S. patent application Ser.No. 2001/0039319 A1 to Harita claims that color irregularities instretched cellulose acetate sheets are reduced when the difference inretardation between widthwise positions within the film is less than 5nm in the original unstretched film. For many applications of opticalfilms, low in-plane retardation values are desirable. In particular,values of in-plane retardation of less than 10 nm are preferred.

Commonly-assigned U.S. Patent Application Publications 2003/0215658A,2003/0215621A, 2003/0215608A, 2003/0215583A, 2003/0215582A,2003/0215581A, 2003/0214715A describe a coating method to prepare resinfilms having low birefringence that are suitable for opticalapplications. The resin films are applied onto a discontinuous,sacrificial substrate from lower viscosity polymer solutions than arenormally used to prepare cast films. Commonly-assigned U.S. patentapplication Ser. No. 10/838,841, filed May 04, 2004 describes a guardedprotective cover sheet having a removable, carrier substrate and a coversheet comprising a low birefringence protective polymer film and a layerpromoting adhesion to poly(vinyl alcohol) on the same side of thecarrier substrate as the low birefringence protective polymer film whicheliminates the need for the saponification process.

Birefringence in cast or coated films arises from orientation ofpolymers during the manufacturing operations. This molecular orientationcauses indices of refraction within the plane of the film to bemeasurably different. In-plane birefringence is the difference betweenthese indices of refraction in perpendicular directions within the planeof the film. The absolute value of birefringence multiplied by the filmthickness is defined as in-plane retardation. Therefore, in-planeretardation is a measure of molecular anisotropy within the plane of thefilm.

During a casting process, molecular orientation may arise from a numberof sources including shear of the dope in the die, shear of the dope bythe metal support during application, shear of the partially dried filmduring the peeling step, and shear of the free-standing film duringconveyance through the final drying step. These shear forces orient thepolymer molecules and ultimately give rise to undesirably highbirefringence or retardation values. To minimize shear and obtain thelowest birefringence films, casting processes are typically operated atvery low line speeds of 1-15 m/min as disclosed in U.S. Pat. No.5,695,694 to Iwata. Slower line speeds generally produce the highestquality films.

Another drawback to the casting method is the inability to accuratelyapply multiple layers. As noted in U.S. Pat. No. 5,256,357 to Hayward,conventional multi-slot casting dies create unacceptably non-uniformfilms. In particular, line and streak non-uniformity is greater than 5%with prior art devices. Acceptable two layer films may be prepared byemploying special die lip designs as taught in U.S. Pat. No. 5,256,357to Hayward, but the die designs are complex and may be impractical forapplying more than two layers simultaneously.

Another drawback to the casting method is the restrictions on theviscosity of the dope. In casting practice, the viscosity of dope is onthe order of 50,000 cp. For example, U.S. Pat. No. 5,256,357 to Haywarddescribes practical casting examples using dopes with a viscosity of100,000 cp. In general, cast films prepared with lower viscosity dopesare known to produce non-uniform films as noted for example in U.S. Pat.No. 5,695,694 to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowestviscosity dopes used to prepare casting samples are approximately 10,000cp. At these high viscosity values, however, casting dopes are difficultto filter and degas. While fibers and larger debris may be removed,softer materials such as polymer slugs are more difficult to filter atthe high pressures found in dope delivery systems. Particulate andbubble artifacts create conspicuous inclusion defects as well as streaksand may create substantial waste.

In addition, the casting method can be relatively inflexible withrespect to product changes. Because casting requires high viscositydopes, changing product formulations requires extensive down time forcleaning delivery systems to eliminate the possibility of contamination.Particularly problematic are formulation changes involving incompatiblepolymers and solvents. In fact, formulation changes are so timeconsuming and expensive with the casting method that most productionmachines are dedicated exclusively to producing only one film type.

Cast films may exhibit undesirable cockle or wrinkles. In addition, manycast films may naturally become distorted over time due to the effectsof moisture. Thinner films are especially vulnerable to dimensionalartifacts either during the peeling and drying steps of the castingprocess or during subsequent handling of the film. In addition, thepreparation of polarizer plates requires a lamination process involvingsaponification pretreatment in an alkali bath and then application ofadhesives, pressure, and high temperatures. Very thin, cast cover sheetsare difficult to handle during this lamination process withoutwrinkling. For optical films, good dimensional stability is necessaryduring storage as well as during subsequent fabrication of polarizerplates. Finally, polymer films used as protective cover sheets forpolarizer plates are susceptible to scratch and abrasion, as well as theaccumulation of dirt and dust, during the manufacture and handling ofthe cover sheet.

SUMMARY OF THE INVENTION

The present invention provides a guarded cover sheet compositecomprising a temporary carrier substrate having a first cover sheetcomprising a first low birefringence film on one side and a second coversheet comprising a second low birefringence film on the other side. Thisinvention further provides a method of making a guarded cover sheetcomposite comprising providing a temporary carrier substrate andapplying a first coating which will form a first low birefringence filmto one side of said carrier substrate; and a second coating which willform a second low birefringence film to the opposite side of saidcarrier substrate, and drying said coatings. It also provides a methodof forming a polarizing plate comprising providing a guarded cover sheetcomposite comprising a temporary carrier substrate having a first lowbirefringence cover sheet on one side and a second low birefringencecover sheet on the other side, peeling the first and second cover sheetsfrom the carrier substrate; providing a dichroic film; andsimultaneously bringing said first and second cover sheets into adhesivecontact with the dichroic film on opposite sides of said dichroic film.

The fabrication of very thin cover sheets is facilitated by the carriersubstrate that supports the wet cover sheet coatings through the dryingprocess and eliminates the need to peel the sheets from a metal band ordrum prior to a final drying step as required in the casting methodsdescribed in prior art. Rather, the cover sheets are completely driedbefore separation from the carrier substrate. In fact, the compositecomprising the cover sheets and carrier substrate are preferably woundinto rolls and stored until needed for the fabrication of polarizerplates. The application of two cover sheets on either side of thecarrier substrate improves the flatness of the cover sheet composite aswell as increasing manufacturing productivity and reducing cost. Inaddition, cover sheets comprising a layer promoting adhesion to PVAdichoric films eliminates the need for saponification of the coversheets prior to lamination to the PVA dichroic film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary coating and drying apparatus thatcan be used in the practice of the method of the present invention.

FIG. 2 is a schematic of an exemplary coating and drying system that canbe used in the practice of the method of the present invention whereinthe cover sheets on either side of the carrier substrate are appliedsequentially.

FIG. 3 is a schematic of an exemplary coating and drying system that canbe used in the practice of the method of the present invention whereinthe cover sheet on either side of the carrier substrate are appliedsimultaneously.

FIG. 4 is a schematic of an exemplary multi-slot coating apparatus thatcan be used in the practice of the present invention.

FIG. 5 shows a cross-sectional representation of a guarded cover sheetcomposite of the invention comprising a multi-layer cover sheet on eachside of a carrier substrate.

FIG. 6 shows a cross-sectional representation of a guarded cover sheetcomposite of the invention comprising a multi-layer cover sheet on eachside of a carrier substrate wherein each cover sheet comprises a layerpromoting adhesion to PVA.

FIG. 7 shows a cross-sectional representation of a guarded cover sheetcomposite of the invention comprising a multi-layer cover sheet on eachside of a carrier substrate wherein the carrier substrate has a releaselayer formed on each side thereon.

FIG. 8 shows a schematic of a method to fabricate a polarizer plateusing the guarded cover sheet composites of the invention.

FIG. 9 shows a cross-sectional representation of a liquid crystal cellwith polarizer plates on either side of the cell.

FIG. 10 is a schematic of a casting apparatus as used in prior art tocast cellulose acetate films.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions apply to the description herein:

In-plane phase retardation R_(in), of a layer is a quantity defined by(nx−ny)d, where nx and ny are indices of refraction in the direction ofx and y. x is taken as a direction of maximum index of refraction in thex-y plane and the y direction is perpendicular to it. The x-y plane isparallel to the surface plane of the layer. d is a thickness of thelayer in the z-direction. The quantity (nx−ny) is referred to asin-plane birefringence, Δn_(in). The value of Δn_(in) is given at awavelength λ=550 nm.

Out of-plane phase retardation R_(th), of a layer is a quantity definedby [nz−(nx+ny)/2]d. nz is the index of refraction in the z-direction.The quantity [nz−(nx+ny)/2] is referred to as out-of-planebirefringence, Δn_(th). If nz>(nx+ny)/2, Δn_(th) is positive (positivebirefringence), thus the corresponding R_(th) is also positive. Ifnz<(nx+ny)/2, Δn_(th) is negative (negative birefringence) and R_(th) isalso negative. The value of Δn_(th) is given at λ=550 nm.

Intrinsic Birefringence Δn_(int) of a polymer refers to the quantitydefined by (ne-no), where ne, and no are the extraordinary and theordinary index of the polymer, respectively. The actual birefringence(in-plane Δn_(in) or out-of-plane Δn_(th)) of a polymer layer depends onthe process of forming it, thus the parameter Δn_(int).

Amorphous means a lack of long-range order. Thus an amorphous polymerdoes not show long-range order as measured by techniques such as X-raydiffraction.

Transmission is a quantity to measure the optical transmissivity. It isgiven by the percentile ratio of out coming light intensity I_(out) toinput light intensity I_(in) as I_(out)/I_(in)×100.

Optic Axis refers to the direction in which propagating light does notsee birefringence.

Uniaxial means that two of the three indices of refraction, nx, ny, andnz, are essentially the same.

Biaxial means that the three indices of refraction, nx, ny, and nz, areall different.

Cover sheets employed in Liquid Crystal Displays are typically polymericsheets having low optical birefringence that are employed on each sideof a dichroic film in order to maintain the dimensional stability of thedichroic film and to protect it from moisture and UV degradation. In thefollowing description a guarded cover sheet means a cover sheet that isdisposed on each side of a temporary carrier substrate. The cover sheetson each side of the carrier substrate are peeled from the carriersubstrate prior to lamination to a PVA dichroic film. After peeling thecover sheets, the carrier substrate may be discarded, recycled, orreused in the manufacture of another guarded cover sheet composite.

A layer promoting adhesion to PVA is a distinct layer that is applied ina coating step either separate from or simultaneous with the applicationof the low birefringence polymer film. The layer promoting adhesion toPVA provides acceptable adhesion of the cover sheet to a PVA dichroicfilm (in a liquid crystal display application) without the need for awet pretreatment, such as saponification, of the cover sheet prior tolamination to the PVA film.

The present invention is directed to an improved cover sheet used in thefabrication of polarizer plates for Liquid Crystal Displays. Inparticular, the present invention provides a guarded cover sheetcomposite comprising a temporary carrier substrate having a first coversheet comprising a first low birefringence film on one side and a secondcover sheet comprising a second low birefringence film on the otherside, optionally, one or both cover sheets further comprise a layerpromoting adhesion to PVA, and/or one or more auxiliary layers. Suitableauxiliary layers for use in the present invention include abrasionresistant hardcoat layer, antiglare layer, anti-smudge layer orstain-resistant layer, antireflection layer, low reflection layer,antistatic layer, viewing angle compensation layer, tie layer, andmoisture barrier layer. The fabrication of very thin cover sheets isfacilitated by the carrier substrate that supports the wet cover sheetcoatings through the drying process and eliminates the need to peel thesheets from a metal band or drum prior to a final drying step asrequired in the casting methods described in prior art. Rather, thecover sheets are completely dried before separation from the carriersubstrate. The application of two cover sheets on either side of thecarrier substrate improves the flatness of the cover sheet composite aswell as increasing manufacturing productivity and reducing cost. Inaddition, cover sheets comprising a layer promoting adhesion to PVAdichroic films eliminates the need for saponification of the coversheets prior to lamination to the PVA dichroic film.

Turning now to FIG. 1 there is shown a schematic of an exemplary andwell-known coating and drying apparatus 10 suitable for preparing theguarded cover sheet composites of the present invention. The coating anddrying apparatus 10 is typically used to apply very thin films to amoving carrier substrate 12 and to subsequently remove solvent in adryer 14. A single coating apparatus 16 is illustrated such thatapparatus 10 has only one coating application point and only one dryer14, however, two or three (even as many as six) additional coatingapplication points with corresponding drying sections are known in thefabrication of composite thin films. The process of sequentialapplication and drying is known in the art as a tandem coatingoperation. In one embodiment of the present invention, a sequential ortandem coating operation may be employed to prepare a guarded coversheet composite whereby a coating and drying apparatus as illustrated inFIG. 1 may be used to apply a first cover sheet onto the carriersubstrate and then a second coating and drying apparatus as illustratedin FIG. 1 may be used to apply a second cover sheet to the other side ofthe carrier substrate.

As shown, coating and drying apparatus 10 includes an unwinding station18 to feed the moving substrate 12 around a back-up roller 20 where thecoating is applied by coating apparatus 16. The coated substrate 22 thenproceeds through the dryer 14. Dryer 14 will typically use airconvection to remove solvent from the coated film. In addition to airconvection, dryer 14 may utilize any other means of supplying energy tothe coating in order to accelerate the drying rate. Examples ofadditional energy sources are infra-red or microwave heating elements.An exemplary dryer 14 used in the practice of the method of the presentinvention includes a first drying section 66 followed by eightadditional drying sections 68-82 capable of independent control oftemperature and air flow. Although dryer 14 is shown as having nineindependent drying sections, drying ovens with fewer compartments arewell known and may be used to practice the method of the presentinvention. In a preferred embodiment of the present invention the dryer14 has at least two independent drying zones or sections.

Preferably, each of drying sections 68-82 each has independenttemperature and airflow controls. In each section, temperature may beadjusted between 5° C. and 150° C. In addition, in each section theairflow characteristics may be altered through the selection of airbaffle geometry and the air volumetric flow rate through the baffles. Tominimize drying defects from case hardening or skinning-over of the wetlayers, optimum drying rates are needed in the early sections of dryer14 and can be achieved through changes in the air temperature andairflow characteristics. There are a number of artifacts created whentemperatures in the early drying zones are inappropriate. For example,fogging or blush of cellulose acetate films is observed when thetemperature in zones 66, 68 and 70 are set below 25° C. This blushdefect is particularly problematic when high vapor pressures solvents(methylene chloride and acetone) are used in the coating fluids.Aggressively high temperatures above 95° C. or aggressively high airimpingement velocities in the early drying sections 66, 68 , and 70 areassociated with artifacts such as case hardening, mottle, reticulationpatterns and blistering of the cover sheet. In preferred embodiment ofthe present invention, the first drying section 66 is operated at atemperature of at least about 25° C. but less than 95° C. with no directair impingement on the wet coating of the coated web 22. In anotherpreferred embodiment of the method of the present invention, dryingsections 68 and 70 are also operated at a temperature of at least about25° C. but less than 95° C. It is preferred that initial drying sections66, 68 be operated at temperatures between about 30° C. and about 60° C.It is most preferred that initial drying sections 66, 68 be operated attemperatures between about 30° C. and about 50° C. The actual dryingtemperature in drying sections 66, 68 and 70 may optimize empiricallywithin these ranges by those skilled in the art.

As an additional means to prevent blush and fogging defects of celluloseacetate films it is necessary to control the air humidity in dryingsections 66-70. In a preferred embodiment of the present invention, thehumidity of the air used in drying sections 66-70 is maintained below adew point temperature of 15° C. It is preferred that the dew pointtemperature of the air used in drying sections 66-70 be below 10° C. Itis most preferred that the dew point temperature of the air used indrying sections 66-70 be below 0° C.

The web path configuration used in drying sections 72-82 is important.Excessive bending of the web can cause premature delamination of thecover sheet from the carrier substrate. In a preferred embodiment of thepresent invention, the minimum bending radius of curvature duringconveyance should be no less than 10 cm. Preferably, the minimum bendingradius of curvature during conveyance should be no less than 25 cm. Itis most preferred that the web path be flat in drying sections 72-82.

As depicted, an exemplary four-layer coating is applied to moving web12. Coating liquid for each layer is held in respective coating supplyvessel 28, 30, 32, 34. The coating liquid is delivered by pumps 36, 38,40, 42 from the coating supply vessels to the coating apparatus 16conduits 44, 46, 48, 50 , respectively. In addition, coating and dryingapparatus 10 may also include electrical discharge devices, such ascorona or glow discharge device 52 , or polar charge assist device 54 ,to modify the substrate 12 prior to application of the coating.

The coating apparatus 16 used to deliver coating fluids to the movingsubstrate 12 may be a multi-layer applicator such as a slide beadhopper, as taught for example in U.S. Pat. No. 2,761,791 to Russell, ora slide curtain hopper, as taught by U.S. Pat. No. 3,508,947 to Hughes.Alternatively, the coating apparatus 16 may be a multi-manifoldextrusion die or a single layer applicator, such as slot die beadhopper, an extrusion die, a wire-wound rod, a knife, an air knife, ablade, a gravure cylinder, a spray nozzle, or jet hopper. In a preferredembodiment of the present invention, the application device 16 is amulti-layer slide bead hopper or a multi-manifold extrusion die.

Turning next to FIG. 2 there is shown a schematic of an exemplarycoating and drying system 5 that essentially comprises a tandem of thecoating and drying apparatus depicted in FIG. 1. However, coating anddrying system 5 further depicts winding station 26 to wind the guardedcover sheet composite 24 into rolls. Accordingly, the drawing in FIG. 2is numbered in an analogous fashion to the drawing in FIG. 1 up to thewinding station, with “a” designating the first coating and dryingapparatus and “b” designating the second coating and drying apparatus.In the practice of the present invention the carrier substrate 12 (whichmay be a resin film, paper, resin coated paper or metal) is suppliedfrom unwind station 18 and conveyed through electrical dischargedevices, such as corona or glow discharge device 52 a, or polar chargeassist device 54 a, to optionally modify the substrate 12 prior toapplication of the coating.

A first cover sheet coating solution is applied at coating apparatus 16a and the coated substrate 22 a then proceeds through the dryer 14 awhere the first coating is dried to form cover sheet coated substrate23. The side of the substrate 23 opposite to that previously coated incoating apparatus 16 a is now conveyed through electrical dischargedevices, such as corona or glow discharge device 52 b, or polar chargeassist device 54 b, to optionally modify the surface prior toapplication of second cover sheet coating solution at coating apparatus16 b. Coated substrate 22 b then proceeds through the dryer 14 b wherethe second coating is dried to form guarded cover sheet composite 24 ,which may be wound into rolls at winding station 26.

Turning next to FIG. 3 there is shown a schematic of an exemplarycoating and drying system 8 suitable for simultaneously coating anddrying a first and second cover sheet on a carrier substrate.Simultaneously coating and drying implies that the both sides of thecarrier substrate are coated at the same time or nearly at the same timeand then both sides of the substrate are dried at the same time in asingle dryer. A suitable coating apparatus to deliver coating fluids toboth sides of the moving substrate 12 at the same time is a duplex typecoating apparatus as taught by U.S. Pat. No. 5,776,251 to Nobuaki.Alternatively, both sides of the substrate can be coating at the sametime using the dip or spray coating methods. Alternatively, both sidesof the substrate may be coated nearly at the same time using two closelyspaced, but separate, coating apparatus. In this latter case a suitablecoating apparatus for the first cover sheet solution is any of thepreviously mentioned coating apparatuses for single or tandem coatingsand a suitable coating apparatus for the second cover sheet solution isany of the previously mentioned coating apparatuses that can be usedwithout a backing roller. Examples of a coating apparatus that can beused without a backing roller and that can instead be operated in a freespan of the web path by control of web tension are a single or multiplemanifold extrusion die, a blade, a knife, an air knife, a spray nozzle,a wire-wound rod, or a gravure cylinder.

After application of the second cover sheet solution the coatedsubstrate 22 then proceeds through the dryer 14 where both sides of thesubstrate are dried to form guarded cover sheet composite 24 , which maybe wound into rolls at winding station 26. It is necessary that anon-contacting web conveyance method be used from the coatingapplication points until the coatings on both sides of the substrate aredried sufficiently so they will not to be damaged by contact. Inparticular, a vertical web path could be used as illustrated in FIG. 3without the need for contact until conveyance element 15. The conveyanceelement in this case could be a roller or an air bar that relies on highair impingement pressure to float the web. In either case it ispreferred that the coating that will come into contact with the rolleror air bar be dried to at least 50% solids by weight prior to thecontact. It is most preferred that the coating be dried to at least 80%solids by weight prior to contacting element 15. It is understood that,in general, dryer 14 could be oriented vertically as shown or in anyother direction and that air impingement nozzles can be used within thedryer to accelerate the drying provided that the coating is not damaged.

Referring now to FIG. 4, a schematic of one exemplary coating apparatus16 is shown in detail. Coating apparatus 16 , schematically shown inside elevational cross-section, includes a front section 92 , a secondsection 94 , a third section 96 , a fourth section 98 , and a back plate100. There is an inlet 102 into second section 94 for supplying coatingliquid to first metering slot 104 via pump 106 to thereby form alowermost layer 108. There is an inlet 110 into third section 96 forsupplying coating liquid to second metering slot 112 via pump 114 toform layer 116. There is an inlet 118 into fourth section 98 forsupplying coating liquid to metering slot 120 via pump 122 to form layer124. There is an inlet 126 into back plate 100 for supplying coatingliquid to metering slot 128 via pump 130 to form layer 132. Each slot104, 112, 120, 128 includes a transverse distribution cavity. Frontsection 92 includes an inclined slide surface 134 , and a coating lip136. There is a second inclined slide surface 138 at the top of secondsection 94. There is a third inclined slide surface 140 at the top ofthird section 96. There is a fourth inclined slide surface 142 at thetop of fourth section 98. Back plate 100 extends above inclined slidesurface 142 to form a back land surface 144. Residing adjacent thecoating apparatus or hopper 16 is a coating backing roller 20 aboutwhich a web 12 is conveyed. Coating layers 108, 116, 124, 132 form amulti-layer composite which forms a coating bead 146 between lip 136 andsubstrate 12. Typically, the coating hopper 16 is movable from anon-coating position toward the coating backing roller 20 and into acoating position. Although coating apparatus 16 is shown as having fourmetering slots, coating dies having a larger number of metering slots(as many as nine or more) are well known and may be used to practice themethod of the present invention.

For the purpose of the present invention, the coating fluids arecomprised principally of a polymer binder dissolved in an organicsolvent. In a particularly preferred embodiment, the low birefringencepolymer film (first, second or both) is a cellulose ester. These arecommercially available in a variety of molecular weight sizes as well asin the type and degree of alkyl substitution of the hydroxyl groups onthe cellulose backbone. Examples of cellulose esters include thosehaving acetyl, propionyl and butyryl groups. Of particular interest isthe family of cellulose esters with acetyl substitution known ascellulose acetate. Of these, the fully acetyl substituted cellulosehaving a combined acetic acid content of approximately 58.0-62.5% isknown as triacetyl cellulose (TAC) and is generally preferred forpreparing cover sheets used in electronic displays.

In terms of organic solvents for TAC, suitable solvents, for example,include chlorinated solvents (methylene chloride and 1,2-dichloroethane), alcohols (methanol, ethanol, n-propanol, isopropanol,n-butanol, isobutanol, diacetone alcohol and cyclohexanol), ketones(acetone, methylethyl ketone, methylisobutyl ketone, and cyclohexanone),esters (methyl acetate, ethyl acetate, n-propyl acetate, isopropylacetate, isobutyl acetate, n-butyl acetate, and methylacetoacetate),aromatics (toluene and xylenes) and ethers (1,3-dioxolane,1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). In someapplications, small amounts of water may be used. Normally, TACsolutions are prepared with a blend of the aforementioned solvents.Preferred primary solvents include methylene chloride, acetone, methylacetate, and 1,3-dioxolane. Preferred co-solvents for use with theprimary solvents include methanol, ethanol, n-butanol and water.

Coating formulations may also contain plasticizers. Appropriateplasticizers for TAC films include phthalate esters (dimethylphthalate,dimethoxyethyl phthalate, diethylphthalate, dibutylphthalate,dioctylphthalate, didecylphthalate and butyl octylphthalate), adipateesters (dioctyl adipate), and phosphate esters (tricresyl phosphate,biphenylyl diphenyl phosphate, cresyl diphenyl phosphate, octyl diphenylphospate, tributyl phosphate, and triphenyl phosphate), glycolic acidesters (triacetin, tributyrin, butyl phthalyl butyl glycolate, ethylphthalyl ethyl glycolate, and methyl phthalyl ethyl glycolate.Plasticizers are normally used to improve the physical and mechanicalproperties of the final film.

In particular, plasticizers are known to improve the flexibility anddimensional stability of cellulose acetate films. However, plasticizersare also used here as coating aids in the converting operation tominimize premature film solidification at the coating hopper and toimprove drying characteristics of the wet film. In the method of thepresent invention, plasticizers are used to minimize blistering, curland delamination of TAC films during the drying operation. In apreferred embodiment of the present invention, plasticizers are added tothe coating fluid at a total concentration of up to 50% by weightrelative to the concentration of polymer in order to mitigate defects inthe final TAC film.

The coating formulation for the low birefringence polymer may alsocontain one or more UV absorbing compounds to provide UV filter elementperformance and/or act as UV stabilizers for the low birefringencepolymer film. Ultraviolet absorbing compounds are generally contained inthe polymer in an amount of 0.01 to 20 weight parts based on 100 weightparts of the polymer containing no ultraviolet absorber, and preferablycontained in an amount of 0.01 to 10 weight parts, especially in anamount of 0.05 to 2 weight parts. Any of the various ultraviolet lightabsorbing compounds which have been described for use in variouspolymeric elements may be employed in the polymeric elements of theinvention, such as hydroxyphenyl-s-triazine, hydroxyphenylbenzotriazole,formamidine, or benzophenone compounds. As described in copending,commonly assigned U.S. patent application Ser. No. 10/150,634, filed May5, 2002, the use of dibenzoylmethane ultraviolet absorbing compounds incombination with a second UV absorbing compound such as those listedabove have been found to be particularly advantageous with respect toproviding both a sharp cut off in absorption between the UV and visiblelight spectral regions as well as increased protection across more ofthe UV spectrum. Additional possible UV absorbers which may be employedinclude salicylate compounds such as 4-t-butylphenylsalicylate; and[2,2′thiobis-(4-t-octylphenolate)]n-butylamine nickel(II). Mostpreferred are combinations of dibenzoylmethane compounds withhydroxyphenyl-s-triazine or hydroxyphenylbenzotriazole compounds.

Dibenzoylmethane compounds which may be employed include those of theformula (IV)

where R1 through R5 are each independently hydrogen, halogen, nitro, orhydroyxl, or further substituted or unsubstituted alkyl, alkenyl, aryl,alkoxy, acyloxy, ester, carboxyl, alkyl thio, aryl thio, alkyl amine,aryl amine, alkyl nitrile, aryl nitrile, arylsulfonyl, or 5-6 memberheterocylce ring groups. Preferably, each of such groups comprises 20 orfewer carbon atoms. Further preferably, R1 through R5 of Formula IV arepositioned in accordance with Formula IV-A:

Particularly preferred are compounds of Formula IV-A where R1 and R5represent alkyl or alkoxy groups of from 1-6 carbon atoms and R2 throughR4 represent hydrogen atoms.

Representative compounds of Formula (IV) which may be employed inaccordance the elements of the invention include the following:

-   (IV-1): 4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane (PARSOL®    1789)-   (IV-2): 4-isopropyl dibenzoylmethane (EUSOLEX® 8020)-   (IV-3): dibenzoylmethane (RHODIASTAB® 83)

Hydroxyphenyl-s-triazine compounds which may be used in the elements ofthe invention, e.g., may be a derivative of tris-aryl-s-triazinecompounds as described in U.S. Pat. No. 4,619,956. Such compounds may berepresented by Formula V:

wherein X, Y and Z are each aromatic, carbocylic radicals of less thanthree 6-membered rings, and at least one of X, Y and Z is substituted bya hydroxy group ortho to the point of attachment to the triazine ring;and each of R^(1a) through R^(9a) is selected from the group consistingof hydrogen, hydroxy, alkyl, alkoxy, sulfonic, carboxy, halo, haloalkyland acylamino. Particularly preferred are hydroxyphenyl-s-triazines ofthe formula V-A:

wherein R is hydrogen or alkyl of 1-18 carbon atoms.

Hydroxyphenylbenzotriazole compounds which may be used in the elementsof the invention, e.g., may be a derivative of compounds represented byFormula VI:

wherein R_(1c) through R_(5c), may be independently hydrogen, halogen,nitro, hydroxy, or further substituted or unsubstituted alkyl, alkenyl,aryl, alkoxy, acyloxy, aryloxy, alkylthio, mono or dialkyl amino, acylamino, or heterocyclic groups. Specific examples of benzotriazolecompounds which may be used in accordance with the invention include2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole;2-(2′-hydroxy-3′,5′-di-t-amylphenyl)benzotriazole; octyl5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzenepropionate;2-(hydroxy-5-t-octylphenyl)benzotriazole;2-(2′-hydroxy-5′-methylphenyl)benzotriazole;2-(2′-hydroxy-3′-dodecyl-5′-methylphenyl)benzotriazole; and2-(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole.

Formamidine compounds which may be used in the elements of theinvention, e.g., may be a formamidine compound as described in U.S. Pat.No. 4,839,405. Such compounds may be represented by Formula VII orFormula VIII:

wherein R_(1d) is an alkyl group containing 1 to about 5 carbon atoms; Yis a H, OH, Cl or an alkoxy group; R_(2d) is a phenyl group or an alkylgroup containing 1 to about 9 carbon atoms; X is selected from the groupconsisting of H, carboalkoxy, alkoxy, alkyl, dialkylamino and halogen;and Z is selected from the group consisting of H, alkoxy and halogen;

wherein A is —COOR, —COOH, —CONR′R″, —NR′COR, —CN, or a phenyl group;and wherein R is an alkyl group of from 1 to about 8 carbon atoms; R′and R″ are each independently hydrogen or lower alkyl groups of from 1to about 4 carbon atoms. Specific examples of formamidine compoundswhich may be used in accordance with the invention include thosedescribed in U.S. Pat. No. 4,839,405, and specifically4-[[(methylphenylamino)methylene]amino]-ethyl ester.

Benzophenone compounds which may be used in the elements of theinvention, e.g., may include 2,2′-dihydroxy-4,4′dimethoxybenzophenone,2-hydroxy-4-methoxybenzophenone and2-hydroxy-4-n-dodecyloxybenzophenone.

Coating formulations may also contain surfactants as coating aids tocontrol artifacts related to flow after coating. Artifacts created byflow after coating phenomena include mottle, repellencies, orange-peel(Bernard cells), and edge-withdraw. Surfactants used control flow aftercoating artifacts include siloxane and fluorochemical compounds.Examples of commercially available surfactants of the siloxane typeinclude:1.) Polydimethylsiloxanes such as DC200 Fluid from Dow Coming,2.) Poly(dimethyl, methylphenyl)siloxanes such as DC510 Fluid from DowComing, and 3.) Polyalkyl substituted polydimethysiloxanes such as DC190and DC1248 from Dow Coming as well as the L7000 Silwet series (L7000,L7001, L7004 and L7230) from Union Carbide, and 4.) Polyalkylsubstituted poly(dimethyl, methylphenyl)siloxanes such as SF1023 fromGeneral Electric. Examples of commercially available fluorochemicalsurfactants include: 1.) Fluorinated alkyl esters such as the Fluoradseries (FC430 and FC431) from the 3M Corporation, 2.) Fluorinatedpolyoxyethylene ethers such as the Zonyl series (FSN, FSN100, FSO,FSO100) from Du Pont, 3.) Acrylate:polyperfluoroalkyl ethylacrylatessuch as the F series (F270 and F600) from NOF Corporation, and 4.)Perfluoroalkyl derivatives such as the Surflon series (S383, S393, andS8405) from the Asahi Glass Company. In the method of the presentinvention, surfactants are generally of the non-ionic type. In apreferred embodiment of the present invention, non-ionic compounds ofeither the siloxane or fluorinated type are added to the uppermostlayers.

In terms of surfactant distribution, surfactants are most effective whenpresent in the uppermost layers of the multi-layer coating. In theuppermost layer, the concentration of surfactant is preferably0.001-1.000% by weight and most preferably 0.010-0.500%. In addition,lesser amounts of surfactant may be used in the second uppermost layerto minimize diffusion of surfactant into the lowermost layers. Theconcentration of surfactant in the second uppermost layer is preferably0.000-0.200% by weight and most preferably between 0.000-0.100% byweight. Because surfactants are only necessary in the uppermost layers,the overall amount of surfactant remaining in the final dried film issmall. In the method of the present invention, a practical surfactantconcentration in the uppermost layer having a wet thickness of 20 μm anda density of 0.93 g/cc is 0.200% by weight which after drying gives afinal surfactant amount of approximately 37 mg/sq-m.

Although surfactants are not required to practice the method of thecurrent invention, surfactants do improve the uniformity of the coatedfilm. In particular, mottle nonuniformities are reduced by the use ofsurfactants. In transparent cellulose acetate films, mottlenonuniformities are not readily visualized during casual inspection. Tovisualize mottle artifacts, organic dyes may be added to the uppermostlayer to add color to the coated film. For these dyed films,nonuniformities are easy to see and quantify. In this way, effectivesurfactant types and levels may be selected for optimum film uniformity.

The preparation of the guarded cover sheet composites of the presentinvention may also include the step of coating over a previouslyprepared composite of low birefringence polymer film and carriersubstrate. For example, the coating and drying apparatus and systemshown in FIGS. 1 through 4 may be used to apply a second multi-layerfilm to an existing low birefringence polymer film/substrate composite.If the film/substrate composite is wound into rolls before applying thesubsequent coating, the process is called a multi-pass coatingoperation. If coating and drying operations are carried out sequentiallyon a machine with multiple coating stations and drying ovens, then theprocess is called a tandem coating operation. In this way, thick filmsmay be prepared at high line speeds without the problems associated withthe removal of large amounts of solvent from a very thick wet film.Moreover, the practice of multi-pass or tandem coating also has theadvantage of minimizing other artifacts such as streak severity, mottleseverity, and overall film nonuniformity.

The prior art method of casting resin films is illustrated in FIG. 11.As shown in FIG. 11, a viscous polymeric dope is delivered through afeed line 200 to an extrusion hopper 202 from a pressurized tank 204 bya pump 206. The dope is cast onto a highly polished metal drum 208located within a first drying section 210 of the drying oven 212. Thecast film 214 is allowed to partially dry on the moving drum 208 and isthen peeled from the drum 208. The cast film 214 is then conveyed to afinal drying section 216 to remove the remaining solvent. The finaldried film 218 is then wound into rolls at a wind-up station 220. Theprior art cast film typically has a thickness in the range of from 40 to200 μpm.

Coating methods are distinguished from casting methods by the processsteps necessary for each technology. These process steps in turn affecta number of tangibles such as fluid viscosity, converting aids,substrates, and hardware that are unique to each method. In general,coating methods involve application of dilute low viscosity liquids tothin flexible substrates, evaporating the solvent in a drying oven, andwinding the dried film/substrate composite into rolls. In contrast,casting methods involve applying a concentrated viscous dope to a highlypolished metal drum or band, partially drying the wet film on the metalsubstrate, stripping the partially dried film from the substrate,removing additional solvent from the partially dried film in a dryingoven, and winding the dried film into rolls. In terms of viscosity,coating methods require very low viscosity liquids of less than 5,000cp. In the present invention the viscosity of the coated liquids willgenerally be less than 2000 cp and most often less than 1500 cp.Moreover, in the present invention the viscosity of the lowermost layeris preferred to be less than 200 cp. and most preferably less than 100cp. for high speed coating application. In contrast, casting methodsrequire highly concentrated dopes with viscosity on the order of10,000-100,000 cp for practical operating speeds. In terms of convertingaids, coating methods generally involve the use of surfactants asconverting aids to control flow after coating artifacts such as mottle,repellencies, orange peel, and edge withdraw. In contrast, castingmethods do not require surfactants. Instead, converting aids are onlyused to assist in the stripping operation in casting methods. Forexample, n-butanol is sometimes used as a converting aid in casting TACfilms to facilitate stripping of the TAC film from the metal drum. Interms of substrates, coating methods generally utilize thin (10-250 μm)flexible supports. In contrast, casting methods employ thick (1-100 mm),continuous, highly polished metal drums or rigid bands. As a result ofthese differences in process steps, the hardware used in coating isconspicuously different from those used in casting as can be seen by acomparison of the schematics shown, for example, in FIGS. 1 and 10,respectively.

Turning next to FIGS. 5 through 7, there are presented cross-sectionalillustrations showing various guarded cover sheet compositeconfigurations possible with the present invention. In FIG. 5, a guardedcover sheet composite 151 comprising two-layer cover sheets 171 and 173on each side of a carrier substrate 170 is shown. First cover sheet 171has lowermost layer 161 , and outermost layer 165 and second cover sheet173 has lowermost layer 162 and outermost layer 166. In thisillustration, layers 161 and 162 could be a first and a second lowbirefringence polymer film, respectively, layer 165 could be a viewingangle compensation layer, and layer 166 could be an abrasion resistanthardcoat layer, for example. Typically, layers 161 and 165 may beapplied together using a multi-layer slide bead hopper as previouslydescribed in FIG. 4. Likewise, layers 162 and 166 may be appliedtogether. Cover sheets 171 and 173 may be formed by simultaneouslyapplying liquid coatings on each side of the carrier substrate 170 andthen drying the layers in a single drying operation, or each side of thecarrier substrate 170 may be coated and dried in a sequential operation.Alternatively, all four of layers 161, 162, 165, and 166 may be coatedand dried in sequential operations.

FIG. 6 illustrates another guarded cover sheet composite 153 comprisinga first cover sheet 175 that is comprised of, for example, threecompositionally discrete layers including a lowermost layer 161 anearest to the carrier substratel 70 , an intermediate layer 163 a, andan uppermost layer 165 a. On the side of the carrier substrate oppositeto first cover sheet 175 is a second cover sheet 177 that is comprisedof, for example, two compositionally discrete layers including alowermost layer 162 a nearest to the carrier substratel 70 and anuppermost layer 166. In this illustration, layers 161 a and 162 a couldbe a layer promoting adhesion to PVA, layers 163 a and 166 a could be afirst and a second low birefringence polymer film, respectively, andlayer 165 a could be an abrasion resistant hardcoat layer, for example.The coating and drying operations that may effectively be employed toform guarded cover sheet composite 153 are analogous to those describedin FIG. 5 above.

FIG. 7 illustrates a further guarded cover sheet composite 155comprising multi-layer cover sheets 179 and 181. First cover sheet 179has lowermost layer 161 b, and outermost layer 165 b and second coversheet 181 has lowermost layer 162 b, intermediate layer 164 b andoutermost layer 166 b. The carrier substrate 182 has been treated oneach side with a release layer 184 to modify the adhesion between thesubstrate 182 and lowermost layers 161 b and 162 b. Release layer 184may be comprised of a number of polymeric materials such aspolyvinylbutyrals, cellulosics, polyacrylates, polycarbonates andpoly(acrylonitrile-co-vinylidene chloride-co-acrylic acid). The choiceof materials used in the release layer may be optimized empirically bythose skilled in the art.

FIGS. 5 through 7 serve to illustrate some of the guarded cover sheetcomposites that may be constructed based on the detailed teachingsprovided hereinabove, they are not intended to be exhaustive of allpossible variations of the invention. One skilled in the art couldconceive of many other layer combinations that would be useful asguarded cover sheet composites for use in the preparation of polarizerplates for LCDs.

Turning now to FIG. 8, a schematic representation of a method tofabricate a polarizer plate from guarded cover sheet composites of theinvention is illustrated. Guarded cover sheet composite 153 (see FIG. 6)comprising first cover sheet 175, second cover sheet 177 and carriersubstrate 170 is supplied from supply roll 232. A PVA-dichroic film 238is supplied from supply roll 236. Prior to entering a lamination nipbetween opposing pinch rollers 242 and 244 , the first cover sheet 175and second cover sheet 177 are peeled from guarded cover sheet composite153 to expose a lowermost layer (in the case of FIGS. 6, this islowermost layers 161 a and 162 a, which for the purpose of example arelayers promoting adhesion to PVA). The cover sheets may be peeledsimultaneously or sequentially, then the bare carrier substrate 170 iswound into a roll at take-up roll 240. A glue solution may be optionallyapplied to both sides of the PVA-dichroic film or to the lowermost layerof cover sheets 175 and 177 prior to the sheets and film entering thenip between pinch rollers 242 and 244. Cover sheets 175 and 177 are thenlaminated to either side of PVA-dichroic film 238 with the applicationof pressure (and, optionally, heat) between the opposing pinch rollers242 and 244 to give the polarizer plate 250. Polarizer plate 250 maythen be dried by heating and wound into rolls until needed. Depending onthe particular layer configuration for the guarded cover sheet compositeemployed, a wide variety of polarizer plates having cover sheets withvarious combinations of auxiliary layers may be fabricated. Preferablythe glue is applied simultaneously with bringing said dichroic film andsaid cover sheets into contact.

In a preferred embodiment of the invention, the cover sheets arelaminated to the PVA dichroic film such that the layer promotingadhesion to PVA is on the side of the cover sheet that contacts the PVAdichroic film. The glue solution useful for laminating the cover filmand the PVA dichroic film is not particularly limited, a commonlyemployed example is a water/alcohol solution containing a dissolvedpolymer such as PVA or its derivatives and a boron compound such asboric acid. Alternatively, the solution may be free or substantiallyfree of dissolved polymer and comprise a reagent that crosslinks PVA.The reagent may crosslink PVA either ionically or covalently or acombination of both types of reagents may be used. Appropriatecrosslinking ions include but are not limited to cations such ascalcium, magnesium, barium, strontium, boron, beryllium, aluminum, iron,copper, cobalt, lead, silver, zirconium and zinc ions. Boron compoundssuch as boric acid and zirconium compounds such as zirconium nitrate orzirconium carbonate are particularly preferred. Examples of covalentcrosslinking reagents include polycarboxylic acids or anhydrides;polyamines; epihalohydrins; diepoxides; dialdehydes; diols; carboxylicacid halides, ketenes and like compounds. The amount of the solutionapplied onto the films can vary widely depending on its composition. Forexample, a wet film coverage as low as 1 cc/m² and as high as 100 cc/m²are possible. Low wet film coverages are desirable to reduce the dryingtime needed.

FIG. 9 presents a cross-sectional illustration showing a liquid crystalcell 260 having polarizer plates 252 and 254 disposed on either side.Polarizer plate 254 is on the side of the LCD cell closest to theviewer. Each polarizer plate employs two cover sheets. For the purposeof illustration, polarizer plate 254 is shown with an uppermost coversheet (this is the cover sheet closest to the viewer) comprising a layerpromoting adhesion to PVA 261, low birefringence polymer film 262,moisture barrier layer 264, antistatic layer 266, and antiglare layer268. The lowermost cover sheet contained in polarizer plate 254comprises a layer promoting adhesion to PVA 261, low birefringencepolymer film 262, moisture barrier layer 264, antistatic layer 266, andviewing angle compensation layer 272. On the opposite side of the LCDcell, polarizer plate 252 is shown with an uppermost cover sheet, whichfor the purpose of illustration, comprises a layer promoting adhesion toPVA 261, low birefringence polymer film 262, moisture barrier layer 264,antistatic layer 266, and viewing angle compensation layer 272.Polarizer plate 252 also has a lowermost cover sheet comprising a layerpromoting adhesion to PVA 261, low birefringence polymer film 262,moisture barrier layer 264, and antistatic layer 266.

Carrier substrates suitable for the use in the present invention includepolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polycarbonate, polystyrene, and other polymeric films. Additionalsubstrates may include paper, laminates of paper and polymeric films,glass, cloth, aluminum and other metal supports. Preferably, the carriersubstrate is a polyester film comprising polyethylene terephthalate(PET) or polyethylene naphthalate (PEN). The thickness of the carriersubstrate is about 10 to 200 micrometers, typically about 10 to 125micrometers, and preferably 10 to 50 micrometers. Thinner carriersubstrates are desirable due to both cost and the weight per roll ofguarded cover sheet composite. A particular benefit of the presentinvention is that by applying a cover sheet on each side of the carriersubstrate the curl of the guarded cover sheet composite product issignificantly reduced compared to single sided application of a coversheet onto a carrier substrate as described in commonly-assigned U.S.patent application Ser. No. 10/838,841. However, even in the presentinvention carrier substrates less than about 10 micrometers may notprovide sufficient dimensional stability or protection for the coversheets.

The carrier substrate may be coated with one or more subbing layers ormay be pretreated with electrical discharge devices to enhance thewetting of the substrate by coating solutions. Since the cover sheetsmust ultimately be peeled from the carrier substrate the adhesionbetween cover sheets and substrate is an important consideration.Subbing layers and electrical discharge devices may also be employed tomodify the adhesion of the cover sheet to the carrier substrate. Subbinglayers may therefore function as either primer layers to improve wettingor release layers to modify the adhesion of the cover sheets to thesubstrate. The carrier substrate may be coated with two subbing layers,the first layer acting as a primer layer to improve wetting and thesecond layer acting as a release layer. The thickness of the subbinglayer is typically 0.05 to 5 micrometers, preferably 0.1 to 1micrometers.

Cover sheet/substrate composites having poor adhesion might be prone toblister after application of a second or third wet coating in amulti-pass operation. To avoid blister defects, adhesion should begreater than about 0.3 N/m between the first-pass layer of the coversheet and the carrier substrate. As already mentioned, the level ofadhesion may be modified by a variety of web treatments includingvarious subbing layers and various electronic discharge treatments.However, excessive adhesion between the cover sheet and substrate isalso undesirable since the cover sheet may be damaged during subsequentpeeling operations. In particular, cover sheet/substrate compositeshaving too great an adhesive force may peel poorly. The maximum adhesiveforce that allows acceptable peel behavior is dependent on the thicknessand tensile properties of the cover sheet. Typically, an adhesive forcebetween the cover sheet and the substrate greater than about 300 N/m maypeel poorly. Cover sheets peeled from such excessively well-adheredcomposites exhibit defects due to tearing of the cover sheet and/or dueto cohesive failure within the sheet. In a preferred embodiment of thepresent invention, the adhesion between the cover sheet and the carriersubstrate is less than 250 N/m. Most preferably, the adhesion betweenthe cover sheet and the carrier substrate is between 0.5 and 25 N/m.

In a one embodiment of the invention, the carrier substrate is apolyethylene terephthalate film having a first subbing layer (primerlayer) comprising a vinylidene chloride copolymer and second subbinglayer (release layer) comprising polyvinyl butyral. In anotherembodiment of the invention the carrier substrate is polyethyleneterephthalate film that has been pretreated with a corona dischargeprior to application of the cover sheets. In a further embodiment of theinvention the cover sheet is applied onto bare and untreatedpolyethylene terephthalate carrier substrate such that the lowermostlayer of each cover sheet is a layer promoting adhesion to PVA.

Substrates may also have functional layers such as antistatic layerscontaining various polymer binders and conductive addenda in order tocontrol static charging and dirt and dust attraction. The antistaticlayer may be on either or both sides of the carrier substrate and alsofunction as a release or subbing layer.

Low birefringence polymer films suitable for use in the presentinvention comprise polymeric materials having low IntrinsicBirefringence Δn_(int) that form high clarity films with high lighttransmission (i.e., >85%). Preferably, the low birefringence polymerfilm has in-plane birefringence, Δinof less than about 1×10⁻⁴ and anout-of-plane birefringence, Δn_(th) of from 0.005 to −0.005.

Exemplary polymeric materials for use in the low birefringence polymerfilms of the invention include cellulose esters (including triacetylcellulose (TAC), cellulose diacetate, cellulose acetate butyrate,cellulose acetate propionate), polycarbonates (such as Lexan® availablefrom General Electric Corp.), polysulfones (such as Udel® available fromAmoco Performance Products Inc.), polyacrylates, and cyclic olefinpolymers (such as Arton® available from JSR Corp., Zeonex® and Zeonor®available from Nippon Zeon, Topas® supplied by Ticona), among others.Preferably, the low birefringence polymer films of the inventioncomprises TAC, polycarbonate, or cyclic olefin polymers due to theircommercial availability and excellent optical properties.

The low birefringence polymer films have a thickness from about 5 to 100micrometers, preferably from about 5 to 50 micrometers and mostpreferably from about 10 to 40 micrometers. Films having thickness of 10to 40 micrometers are most preferred due to cost, handling, ability toprovide thinner polarizer plates, and improved light transmission.Polarizer plates fabricated from conventional cover sheets comprising alow birefringence polymer film thickness of about 80 micrometers have atotal thickness of at least 180 micrometers. In a preferred embodimentof the current invention, polarizer plates assembled from cover sheetsof the invention have a total thickness of less than 120 micrometers,and most preferably less than 80 micrometers.

In accordance with the present invention the thickness and thecomposition of the first low birefringence polymer film may be the sameor different from the second low birefringence polymer film. Thethickness and composition of each low birefringence polymer film will bechosen in order to satisfy the design and performance requirements forthe polarizer plate being fabricated and/or provide the needed flatnessor freedom from film curl for the guarded cover sheet composite.

Materials useful for formning a layer promoting adhesion to PVA aretypically water-swellable, hydrophilic polymers which include bothsynthetic and natural polymers. Naturally occurring substances includeproteins, protein derivatives, cellulose derivatives (e.g. celluloseesters), polysaccharides, casein, and the like, and synthetic polymersinclude poly(vinyl lactams), acrylamide polymers, polyvinyl alcohol andits derivatives, hydrolyzed polyvinyl acetates, polymers of alkyl andsulfoalkyl acrylates and methacrylates, polyamides, polyvinyl pyridine,acrylic acid polymers, maleic anhydride copolymers, polyalkylene oxide,methacrylamide copolymers, polyvinyl oxazolidinones, maleic acidcopolymers, vinyl amine copolymers, methacrylic acid copolymers,acryloyloxyalkyl sulfonic acid copolymers, vinyl imidazole copolymers,vinyl sulfide copolymers, homopolymer or copolymers containing styrenesulfonic acid, and the like. The most preferred polymers are polyvinylalcohol and its derivatives.

Other suitable polymers useful in the layer promoting adhesion to PVAinclude water dispersible polymers or polymer latexes. Preferably thesewater dispersible polymers contain at least one hydrophilic moiety,which includes hydroxyl, carboxyl, amino, or sulfonyl moieties. Suchpolymers include addition-type polymers and interpolymers prepared fromethylenically unsaturated monomers such as acrylates including acrylicacid, methacrylates including methacrylic acid, acrylamides andmethacrylamides, itaconic acid and its half esters and diesters,styrenes including substituted styrenes, acrylonitrile andmethacrylonitrile, vinyl acetates, vinyl ethers, vinyl and vinylidenehalides, and olefins. In addition, crosslinking and graft-linkingmonomers such as 1,4-butyleneglycol methacrylate, trimethylolpropanetriacrylate, allyl methacrylate, diallyl phthalate, divinyl benzene, andthe like may be used. Other suitable polymer dispersions arepolyurethane dispersions or polyesterionomer dispersions,polyurethane/vinyl polymer dispersions, fluoropolymer dispersions. Thesepolymer dispersions have a particle size in the range of from 10nanometers to 1 micron.

The layer promoting adhesion to PVA may also contain a crosslinkingagent. Crosslinking agents useful for the practice of the inventioninclude any compounds that are capable of reacting with the hydrophilicmoieties attached to the polymer binder. Such crosslinking agentsinclude aldehydes and related compounds, pyridiniums, olefins such asbis(vinylsulfonyl methyl) ether, carbodiimides, epoxides, triazines,polyfunctional aziridines, methoxyalkyl melamines, polyisocyanates, andthe like. These compounds can be readily prepared using the publishedsynthetic procedure or routine modifications that would be readilyapparent to one skilled in the art of synthetic organic chemistry.Additional crosslinking agents that may also be successfully employed inthe layer promoting adhesion to PVA include multivalent metal ion suchas zinc, calcium, zirconium and titanium.

The layer promoting adhesion to PVA may also be an optically clear,pressure sensitive adhesive layer. A wide variety of these pressuresensitive adhesives are available. Adhesive materials useful forlaminating the cover sheet to the PVA dichroic film can be selected fromthe general class of “modified acrylics” that have good adhesion, aretransparent, and are inert with respect to chemical and UV aging andyellowing. High strength adhesives useful in this invention, forexample, are aqueous-based adhesives such as Aeroset® 2177 or Aeroset®t2550, 3240, and 3250 which are commercially available from AshlandChemical Co., PD0681, AP6903, and W3320 available from H. B. Fuller, orsolvent-based pressure sensitive adhesives such as PS508 sold by AshlandChemical Co. The adhesives may be used separately or in combination.

The layer promoting adhesion to PVA is typically applied at a driedcoating thickness of 0.1 to 5 micrometers, preferably 0.25 to 1micrometers. The layer promoting adhesion to PVA may be on either sideof the cover sheet relative to the low birefringence film. It may beadjacent to the carrier substrate or it may be on the side of the lowbirefringence film opposite to the carrier substrate.

Preferably, the layer promoting adhesion to PVA is between the carriersubstrate and the low birefringence film. Most preferably, the layerpromoting adhesion to PVA is applied directly onto the carrier substrateor onto a subbing layer on the carrier substrate. The layer promotingadhesion to PVA may be coated in a separate coating application or itmay be applied simultaneously with one or more other layers.

Liquid Crystal Displays typically employ two polarizer plates, one oneach side of the liquid crystal cell. Each polarizer plate, in turn,employs two cover sheets, one on each side of the PVA-dichroic film.Each cover sheet may have various auxiliary layers that are necessary toimprove the performance of the Liquid Crystal Display. Useful auxiliarylayers employed in the cover sheets of the invention include: abrasionresistant hardcoat layer, antiglare layer, anti-smudge layer orstain-resistant layer, antireflection layer, low reflection layer,antistatic layer, viewing angle compensation layer, tie layer, andmoisture barrier layer. Typically, the cover sheet closest to the viewercontains one or more of the following auxiliary layers: the abrasionresistant layer, anti-smudge or stain-resistant layer, antireflectionlayer, and antiglare layer. One or both of the cover sheets closest tothe liquid crystal cell typically contain a viewing angle compensationlayer. Any or all of the four cover sheets employed in the LCD mayoptionally contain tie layer, an antistatic layer and a moisture barrierlayer.

In accordance with the present invention, a tie layer is advantageouslyemployed between the layer promoting adhesion to PVA and the lowbirefringence polymer film to insure excellent adhesion of thehydrophilic layer promoting adhesion to PVA and the more hydrophobic lowbirefingence polymer film. The tie layer comprises, preferably in anamount of at least 50 weight %, of a polymer having an acid number ofbetween 20 and 300, preferably 50 to 200. It is suitably soluble in avariety of common organic solvents at 20° C. The acid functionality is acarboxylic acid (a carboxy group, also known as a carboxyl group).Polymers suitable for use in the tie layer include copolymers (includinginterpolymers) of ethylenically unsaturated monomers comprisingcarboxylic acid groups, acid-containing cellulosic polymers such ascellulose acid phthalate and cellulose acetate trimellitate,polyurethanes having carboxylic acid groups, and others. Suitablecopolymers of ethylenically unsaturated monomers comprising carboxylicacid groups include acrylates including acrylic acid, methacrylatesincluding methacrylic acid, acrylamides and methacrylamides, itaconicacid and its half esters and diesters, styrenes including substitutedstyrenes, acrylonitrile and methacrylonitrile, vinyl acetates, vinylethers, vinyl and vinylidene halides, and olefins. Preferably, the glasstransition temperature of the carboxy-functional polymer is greater than20° C.

Organic solvents suitable for solubilizing and coating the tie layerpolymer include chlorinated solvents (methylene chloride and 1,2dichloroethane), alcohols (methanol, ethanol, n-propanol, isopropanol,n-butanol, isobutanol, diacetone alcohol and cyclohexanol), ketones(acetone, methylethyl ketone, methylisobutyl ketone, and cyclohexanone),esters (methyl acetate, ethyl acetate, n-propyl acetate, isopropylacetate, isobutyl acetate, n-butyl acetate, and methylacetoacetate),aromatics (toluene and xylenes) and ethers (1,3-dioxolane,1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). Preferablythe tie-layer polymer described above is essentially soluble in at leastone, preferably most of the above 28 named solvents, more preferablysoluble in at least one of the solvents in most of the six mentionedgroups (chlorinated, alcohols, etc.). In some applications, smallamounts of water may be used. Normally, the coating solutions areprepared with a blend of the aforementioned solvents. Preferred primarysolvents include methylene chloride, acetone, methyl acetate, and1,3-dioxolane. Preferably, the tie-layer polymer is substantiallysoluble in these solvents. Preferred co-solvents for use with theprimary solvents include methanol, ethanol, n-butanol and water.Preferably, the tie layer polymer is applied from the same or at leastcompatible solvent mixture as the low birefringence polymer film. Ingeneral, solubility refers to greater than 1.0 weight percent,preferably at least 2.0 percent, at 20° C.

The tie layer may also contain a crosslinking agent. Crosslinking agentsuseful for the practice of the invention include any compounds that arecapable of reacting with reactive moieties present on the polymer,particularly carboxylic acid. Such crosslinking agents includeboron-containing compounds such as borates, aldehydes and relatedcompounds, pyridiniums, olefins such as bis(vinylsulfonyl methyl) ether,carbodiimides, polyfunctional epoxides, triazines, polyfunctionalaziridines, methoxyalkyl melamines, melamine-formaldehyde resins,polyisocyanates, and the like, or mixtures thereof. These compounds canbe readily prepared using the published synthetic procedure or routinemodifications that would be readily apparent to one skilled in the artof synthetic organic chemistry. Additional crosslinking agents that mayalso be successfully employed in the layer include multivalent metal ionsuch as zinc, calcium, zirconium and titanium.

The tie layer is typically applied at a dried coating weight of 5 to 500mg/ft² (50 to 5000 mg/m²), preferably 50 to 500 mg/ft² (500 to 5000mg/m²) and has a thickness of preferably 0.5 to 5 micrometers. The layeris highly transparent and, preferably, has a light transmission ofgreater than 95%.

Generally, the tie layer is applied onto an already coated and driedlayer promoting adhesion to PVA. The tie layer may be coated in aseparate coating application or it may be applied simultaneously withone or more other layers. Preferably, for best adherence, the tie layeris applied simultaneously with the low birefringence polymer film.

Particularly effective abrasion resistant layers for use in the presentinvention comprise radiation or thermally cured compositions, andpreferably the composition is radiation cured. Ultraviolet (UV)radiation and electron beam radiation are the most commonly employedradiation curing methods. UV curable compositions are particularlyuseful for creating the abrasion resistant layer of this invention andmay be cured using two major types of curing chemistries, free radicalchemistry and cationic chemistry. Acrylate monomers (reactive diluents)and oligomers (reactive resins and lacquers) are the primary componentsof the free radical based formulations, giving the cured coating most ofits physical characteristics. Photo-initiators are required to absorbthe UV light energy, decompose to form free radicals, and attack theacrylate group C═C double bond to initiate polymerization. Cationicchemistry utilizes cycloaliphatic epoxy resins and vinyl ether monomersas the primary components. Photo-initiators absorb the UV light to forma Lewis acid, which attacks the epoxy ring initiating polymerization. ByUV curing is meant ultraviolet curing and involves the use of UVradiation of wavelengths between 280 and 420 nm preferably between 320and 410 nm.

Examples of UV radiation curable resins and lacquers usable for theabrasion layer useful in this invention are those derived from photopolymerizable monomers and oligomers such as acrylate and methacrylateoligomers (the term “(meth)acrylate” used herein refers to acrylate andmethacrylate), of polyfunctional compounds, such as polyhydric alcoholsand their derivatives having (meth)acrylate functional groups such asethoxylated trimethylolpropane tri(meth)acrylate, tripropylene glycoldi(meth)acrylate, trimethylolpropane tri(meth)acrylate, diethyleneglycol di(meth)acrylate, pentaerythritol tetra(meth)acrylate,pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate,1,6-hexanediol di(meth)acrylate, or neopentyl glycol di(meth)acrylateand mixtures thereof, and acrylate and methacrylate oligomers derivedfrom low-molecular weight polyester resin, polyether resin, epoxy resin,polyurethane resin, alkyd resin, spiroacetal resin, epoxy acrylates,polybutadiene resin, and polythiol-polyene resin, and the like andmixtures thereof, and ionizing radiation-curable resins containing arelatively large amount of a reactive diluent. Reactive diluents usableherein include monofunctional monomers, such as ethyl (meth)acrylate,ethylhexyl (meth)acrylate, styrene, vinyltoluene, andN-vinylpyrrolidone, and polyfunctional monomers, for example,trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate,tripropylene glycol di(meth)acrylate, diethylene glycoldi(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritolhexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, or neopentyl glycoldi(meth)acrylate.

Among others, in the present invention, conveniently used radiationcurable lacquers include urethane (meth)acrylate oligomers. These arederived from reacting diisocyanates with a oligo(poly)ester oroligo(poly)ether polyol to yield an isocyanate terminated urethane.Subsequently, hydroxy terminated acrylates are reacted with the terminalisocyanate groups. This acrylation provides the unsaturation to the endsof the oligomer. The aliphatic or aromatic nature of the urethaneacrylate is determined by the choice of diisocyanates. An aromaticdiisocyanate, such as toluene diisocyanate, will yield an aromaticurethane acrylate oligomer. An aliphatic urethane acrylate will resultfrom the selection of an aliphatic diisocyanate, such as isophoronediisocyanate or hexyl methyl diisocyanate. Beyond the choice ofisocyanate, polyol backbone plays a pivotal role in determining theperformance of the final the oligomer. Polyols are generally classifiedas esters, ethers, or a combination of these two. The oligomer backboneis terminated by two or more acrylate or methacrylate units, which serveas reactive sites for free radical initiated polymerization. Choicesamong isocyanates, polyols, and acrylate or methacrylate terminationunits allow considerable latitude in the development of urethaneacrylate oligomers. Urethane acrylates like most oligomers, aretypically high in molecular weight and viscosity. These oligomers aremultifunctional and contain multiple reactive sites. Because of theincreased number of reactive sites, the cure rate is improved and thefinal product is cross-linked. The oligomer functionality can vary from2 to 6.

Among others, conveniently used radiation curable resins includepolyfunctional acrylic compounds derived from polyhydric alcohols andtheir derivatives such as mixtures of acrylate derivatives ofpentaerythritol such as pentaerythritol tetraacrylate andpentaerythritol triacrylate functionalized aliphatic urethanes derivedfrom isophorone diisocyanate. Some examples of urethane acrylateoligomers used in the practice of this invention that are commerciallyavailable include oligomers from Sartomer Company (Exton, PA). Anexample of a resin that is conveniently used in the practice of thisinvention is CN968® from Sartomer Company.

A photo polymerization initiator, such as an acetophenone compound, abenzophenone compound, Michler's benzoyl benzoate, α-amyloxime ester, ora thioxanthone compound and a photosensitizer such as n-butyl amine,triethylamine, or tri-n-butyl phosphine, or a mixture thereof isincorporated in the ultraviolet radiation curing composition. In thepresent invention, conveniently used initiators are 1-hydroxycyclohexylphenyl ketone and 2-methyl-1-[4-(methyl thio)phenyl]-2-morpholinopropanone-1.

The abrasion resistant layer is typically applied after coating anddrying the low birefringence polymer film. The abrasion resistant layerof this invention is applied as a coating composition that typicallyalso includes organic solvents. Preferably the concentration of organicsolvent is 1-99% by weight of the total coating composition.

Examples of solvents employable for coating the abrasion resistant layerof this invention include solvents such as methanol, ethanol, propanol,butanol, cyclohexane, heptane, toluene and xylene, esters such as methylacetate, ethyl acetate, propyl acetate and mixtures thereof. With theproper choice of solvent, adhesion of the abrasion resistant layer canbe improved while minimizing migration of plasticizers and other addendafrom the low birefringence polymer film, enabling the hardness of theabrasion resistant layer to be maintained. Suitable solvents for TAC lowbirefringence polymer film are aromatic hydrocarbon and ester solventssuch as toluene and propyl acetate.

The UV polymerizable monomers and oligomers are coated and dried, andsubsequently exposed to UV radiation to form an optically clearcross-linked abrasion resistant layer. The preferred UV cure dosage isbetween 50 and 1000 mJ/cm².

The thickness of the abrasion resistant layer is generally about 0.5 to50 micrometers preferably 1 to 20 micrometers, more preferably 2 to 10micrometers.

The abrasion resistant layer is preferably colorless, but it isspecifically contemplated that this layer can have some color for thepurposes of color correction, or for special effects, so long as it doesnot detrimentally affect the formation or viewing of the display throughthe overcoat. Thus, there can be incorporated into the polymer dyes thatwill impart color. In addition, additives can be incorporated into thepolymer that will give to the layer desired properties. Other additionalcompounds may be added to the coating composition, includingsurfactants, emulsifiers, coating aids, lubricants, matte particles,rheology modifiers, crosslinking agents, antifoggants, inorganic fillerssuch as conductive and nonconductive metal oxide particles, pigments,magnetic particles, biocide, and the like.

The abrasion resistant layer of the invention typically provides a layerhaving a pencil hardness (using the Standard Test Method for Hardness byPencil Test ASTM D3363) of at least 2H and preferably 2H to 8H.

The cover sheets of the invention may contain an antiglare layer, a lowreflection layer or an antireflection layer on the same side of thecarrier substrate as the low birefringence polymer film. Preferably, theantiglare layer, low reflection layer or antireflection layer is locatedon the side of the low birefringence polymer film opposite to thecarrier substrate. Such layers are employed in an LCD in order toimprove the viewing characteristics of the display, particularly when itis viewed in bright ambient light. The refractive index of an abrasionresistant, hard coat is about 1.50, while the index of the surroundingair is 1.00. This difference in refractive index produces a reflectionfrom the surface of about 4%.

An antiglare coating provides a roughened or textured surface that isused to reduce specular reflection. All of the unwanted reflected lightis still present, but it is scattered rather than specularly reflected.For the purpose of the present invention, the antiglare coatingpreferably comprises a radiation cured composition that has a texturedor roughened surface obtained by the addition of organic or inorganic(matting) particles or by embossing the surface. The radiation curedcompositions described hereinabove for the abrasion resistant layer arealso effectively employed in the antiglare layer. Surface roughness ispreferably obtained by the addition of matting particles to theradiation cured composition. Suitable particles include inorganiccompounds having an oxide, nitride, sulfide or halide of a metal, metaloxides being particularly preferred. As the metal atom, Na, K, Mg, Ca,Ba, Al, Zn, Fe, Cu, Ti, Sn, In, W, Y, Sb, Mn, Ga, V, Nb, Ta, Ag, Si, B,Bi, Mo, Ce, Cd, Be, Pb and Ni are suitable, and Mg, Ca, B and Si aremore preferable. An inorganic compound containing two types of metal mayalso be used. A particularly preferable inorganic compound is silicondioxide, namely silica.

Additional particles suitable for use in the antiglare layer of thepresent invention include the layered clays described incommonly-assigned U.S. patent application Ser. No. 10/690,123, filedOct. 21, 2003. The most suitable layered particles include materials inthe shape of plates with high aspect ratio, which is the ratio of a longdirection to a short direction in an asymmetric particle. Preferredlayered particles are natural clays, especially natural smectite claysuch as montmorillonite, nontronite, beidellite, volkonskoite,hectorite, saponite, sauconite, sobockite, stevensite, svinfordite,halloysite, magadiite, kenyaite and vermiculite as well as layereddouble hydroxides or hydrotalcites. Most preferred clay materialsinclude natural montmorillonite, hectorite and hydrotalcites, because ofcommercial availability of these materials.

The layered materials suitable for this invention may comprisephyllosilicates, for example, montmorillonite, particularly sodiummontmorillonite, magnesium montmorillonite, and/or calciummontmorillonite, nontronite, beidellite, volkonskoite, hectorite,saponite, sauconite, sobockite, stevensite, svinfordite, vermiculite,magadiite, kenyaite, talc, mica, kaolinite, and mixtures thereof Otheruseful layered materials may include illite, mixed layeredillite/smectite minerals, such as ledikite and admixtures of illiteswith the layered materials named above. Other useful layered materials,particularly useful with anionic matrix polymers, may include thelayered double hydroxide clays or hydrotalcites, such asMg₆Al_(3.4)(OH)_(18.8)(CO₃)_(1.7)H₂O, which have positively chargedlayers and exchangeable anions in the interlayer spaces. Preferredlayered materials are swellable so that other agents, usually organicions or molecules, may splay, that is, intercalate and/or exfoliate, thelayered material resulting in a desirable dispersion of the inorganicphase. These swellable layered materials include phyllosilicates of the2:1 type, as defined in the literature (for example, “An introduction toclay colloid chemistry,” by H. van Olphen, John Wiley & SonsPublishers). Typical phyllosilicates with ion exchange capacity of 50 to300 milliequivalents per 100 grams are preferred. Generally, it isdesirable to treat the selected clay material to separate theagglomerates of platelet particles to small crystals, also calledtactoids, prior to introducing the platelet particles to the antiglarecoating. Predispersing or separating the platelet particles alsoimproves the binder/platelet interface. Any treatment that achieves theabove goals may be used. Examples of useful treatments includeintercalation with water soluble or water insoluble polymers, organicreagents or monomers, silane compounds, metals or organometallics,organic cations to effect cation exchange, and their combinations.

Additional particles for use in the antiglare layer of the presentinvention include polymer matte particles or beads which are well knownin the art. The polymer particles may be solid or porous, preferablythey are crosslinked polymer particles. Porous polymer particles for usein an antiglare layer are described in commonly-assigned U.S. patentapplication Ser. No. 10/715,706, filed Nov. 18, 2003.

Particles for use in the antiglare layer have an average particle sizeranging from 2 to 20 micrometers, preferably from 2 to 15 micrometersand most preferably from 4 to 10 micrometers. They are present in thelayer in an amount of at least 2 wt percent and less than 50 percent,typically from about 2 to 40 wt. percent, preferably from 2to 20 percentand most preferably from 2 to 10 percent.

The thickness of the antiglare layer is generally about 0.5 to 50micrometers preferably 1 to 20 micrometers more preferably 2 to 10micrometers.

Preferably, the antiglare layer used in the present invention has a 60°Gloss value, according to ASTM D523, of less than 100, preferably lessthan 90 and a transmission haze value, according to ASTM D-1003 and JISK-7105 methods, of less than 50%, preferably less than 30%.

In another embodiment of the present invention, a low reflection layeror antireflection layer is used in combination with an abrasionresistant hard coat layer or antiglare layer. The low reflection orantireflection coating is applied on top of the abrasion resistant orantiglare layer. Typically, a low reflection layer provides an averagespecular reflectance (as measured by a spectrophotometer and averagedover the wavelength range of 450 to 650 nm) of less than 2%.Antireflection layers provide average specular reflectance values ofless than 1%.

Suitable low reflection layers for use in the present invention comprisefluorine-containing homopolymers or copolymers having a refractive indexof less than 1.48, preferably with a refractive index between about 1.35and 1.40. Suitable fluorine-containing homopolymers and copolymersinclude: fluoro-olefins (for example, fluoroethylene, vinylidenefluoride, tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene,perfluoro-2,2-dimethyl-1,3-dioxol), partially or completely fluorinatedalkyl ester derivatives of (meth)acrylic acid, and completely orpartially fluorinated vinyl ethers, and the like. The effectiveness ofthe layer may be improved by the incorporation of submicron-sizedinorganic particles or polymer particles that induce interstitial airvoids within the coating. This technique is further described in U.S.Pat. No. 6,210,858 and U.S. Pat. No. 5,919,555. Further improvement ofthe effectiveness of the low reflection layer may be realized with therestriction of air voids to the internal particle space ofsubmicron-sized polymer particles with reduced coating haze penalty, asdescribed in commonly-assigned U.S. patent application Ser. No.10/715,655, filed Nov. 18, 2003.

The thickness of the low reflection layer is 0.01 to 1 micrometer andpreferably 0.05 to 0.2 micrometer.

An antireflection layer may comprise a monolayer or a multi-layer.Antireflection layers comprising a monolayer typically providereflectance values less than 1% at only a single wavelength (within thebroader range of 450 to 650 nm). A commonly employed monolayerantireflection coating that is suitable for use in the present inventioncomprises a layer of a metal fluoride such as magnesium fluoride (MgF₂).The layer may be applied by well-known vacuum deposition technique or bya sol-gel technique. Typically, such a layer has an optical thickness(i.e., the product of refractive index of the layer times layerthickness) of approximately one quarter-wavelength at the wavelengthwhere a reflectance minimum is desired.

Although a monolayer can effectively reduce the reflection of lightwithin a very narrow wavelength range, more often a multi-layercomprising several (typically, metal oxide based) transparent layerssuperimposed on one another is used to reduce reflection over a widewavelength region (i.e., broadband reflection control). For such astructure, half wavelength layers are alternated with quarter wavelengthlayers to improve performance. The multi-layer antireflection coatingmay comprise two, three, four, or even more layers. Formation of thismulti-layer typically requires a complicated process comprising a numberof vapor deposition procedures or sol-gel coatings, which correspond tothe number of layers, each layer having a predetermined refractive indexand thickness. Precise control of the thickness of each layer isrequired for these interference layers. The design of suitablemulti-layer antireflection coatings for use in the present invention iswell known in the patent art and technical literature, as well as beingdescribed in various textbooks, for example, in H. A. Macleod, “ThinFilm Optical Filters,” Adam Hilger, Ltd., Bristol 1985 and James D.Rancourt, “Optical Thin Films User's Handbook”, Macmillan PublishingCompany, 1987.

The cover sheets of the invention may contain a moisture barrier on oneor both sides of the low birefringence polymer film. The moisturebarrier layer comprises a hydrophobic polymer such as a vinylidenechloride polymer, vinylidene fluoride polymer, polyurethane, polyolefin,fluorinated polyolefin, polycarbonate, and others, having a low moisturepermeability. Preferably, the hydrophobic polymer comprises vinylidenechloride. More preferably, the hydrophobic polymer comprises 70 to 99weight percent of vinylidene chloride. The moisture barrier layer may beapplied by application of an organic solvent-based or aqueous coatingformulation. To provide effective moisture barrier properties the layershould be at least I micrometer in thickness, preferably from 1 to 10micrometers in thickness, and most preferably from 2 to 8 micrometers inthickness. The cover sheet of the invention comprising a moisturebarrier layer has a moisture vapor transmission rate (MVTR) according toASTM F-1249 that is less than 1000 g/m²/day, preferably less than 800g/m²/day and most preferably less than 500 g/m²/day. The use of such abarrier layer in the cover sheet of the invention provides improvedresistance to changes in humidity and increased durability of thepolarizer comprising the cover sheet, especially for TAC cover sheetshaving a thickness less than about 40 micrometers.

The cover sheets of the invention may contain a transparent antistaticlayer on either side of the low birefringence polymer film. Theantistatic layer aids in the control of static charging that may occurduring the manufacture and use of the cover sheet composite. Effectivecontrol of static charging reduces the propensity for the attraction ofdirt and dust to the cover sheet composite. The guarded cover sheetcomposite of the invention may be particularly prone to triboelectriccharging during the peeling of the cover sheet from the carriersubstrate. The so-called “separation charge” that results from theseparation of the cover sheet and the substrate can be effectivelycontrolled by an antistatic layer having a resistivity of less thanabout 1×10¹¹ Ω/square, preferably less than 1×10¹⁰ Ω/square, and mostpreferably less than 1×10⁹ Ω/square.

Various polymeric binders and conductive materials may be employed inthe antistatic layer. Polymeric binders useful in the antistatic layerinclude any of the polymers commonly used in the coating art, forexample, interpolymers of ethylenically unsaturated monomers, cellulosederivatives, polyurethanes, polyesters, hydrophilic colloids such asgelatin, polyvinyl alcohol, polyvinyl pyrrolidone, and others.

Conductive materials employed in the antistatic layer may be eitherionically-conductive or electronically-conductive. lonically-conductivematerials include simple inorganic salts, alkali metal salts ofsurfactants, polymeric electrolytes containing alkali metal salts, andcolloidal metal oxide sols (stabilized by metal salts). Of these,ionically-conductive polymers such as anionic alkali metal salts ofstyrene sulfonic acid copolymers and cationic quaternary ammoniumpolymers of U.S. Pat. No. 4,070,189 and ionically-conductive colloidalmetal oxide sols which include silica, tin oxide, titania, antimonyoxide, zirconium oxide, alumina-coated silica, alumina, boehmite, andsmectite clays are preferred.

The antistatic layer employed in the current invention preferablycontains an electronically-conductive material due to their humidity andtemperature independent conductivity. Suitable materials include:

-   1) electronically-conductive metal-containing particles including    donor-doped metal oxides, metal oxides containing oxygen    deficiencies, and conductive nitrides, carbides, and bromides.    Specific examples of particularly useful particles include    conductive SnO₂, In₂O, ZnSb₂O₆, InSbO₄, TiB₂, ZrB₂, NbB₂, TaB₂, CrB,    MoB, WB, LaB₆, ZrN, TiN, WC, HfC, HfN, and ZrC. Examples of the    patents describing these electrically conductive particles include;    U.S. Pat. Nos 4,275,103; 4,394,441; 4,416,963; 4,418,141; 4,431,764;    4,495,276; 4,571,361; 4,999,276; 5,122,445; and 5,368,995.-   2) fibrous electronic conductive particles comprising, for example,    antimony-doped tin oxide coated onto non-conductive potassium    titanate whiskers as described in U.S. Pat. Nos. 4,845,369 and    5,166,666, antimony-doped tin oxide fibers or whiskers as described    in U.S. Pat. Nos. 5,719,016 and 5,0731,119, and the silver-doped    vanadium pentoxide fibers described in U.S. Pat. No. 4,203,769-   3) electronically-conductive polyacetylenes, polythiophenes, and    polypyrroles, preferably the polyethylene dioxythiophene described    in U.S. Pat. No. 5,370,981 and commercially available from Bayer    Corp. as Baytron® P.

The amount of the conductive agent used in the antistatic layer of theinvention can vary widely depending on the conductive agent employed.For example, useful amounts range from about 0.5 mg/m² to about 1000mg/m², preferably from about 1 mg/m² to about 500 mg/m². The antistaticlayer has a thickness of from 0.05 to 5 micrometers, preferably from 0.1to 0.5 micrometers to insure high transparency.

Contrast, color reproduction, and stable gray scale intensities areimportant quality attributes for electronic displays, which employliquid crystal technology. The primary factor limiting the contrast of aliquid crystal display is the propensity for light to “leak” throughliquid crystal elements or cells, which are in the dark or “black” pixelstate. Furthermore, the leakage and hence contrast of a liquid crystaldisplay are also dependent on the direction from which the displayscreen is viewed. Typically the optimum contrast is observed only withina narrow viewing angle range centered about the normal incidence to thedisplay and falls off rapidly as the viewing direction deviates from thedisplay normal. In color displays, the leakage problem not only degradesthe contrast but also causes color or hue shifts with an associateddegradation of color reproduction.

Thus, one of the major factors measuring the quality of LCDs is theviewing angle characteristic, which describes a change in contrast ratiofrom different viewing angles. It is desirable to be able to see thesame image from a wide variation in viewing angles and this ability hasbeen a shortcoming with liquid crystal display devices. One way toimprove the viewing angle characteristic is to employ a cover sheethaving a viewing angle compensation layer (also referred to as acompensation layer, retarder layer, or phase difference layer), withproper optical properties, between the PVA-dichroic film and liquidcrystal cell, such as disclosed in U.S. Pat. Nos. 5,583,679, 5,853,801,5,619,352, 5,978,055, and 6,160,597. A compensation film according toU.S. Pat. Nos. 5,583,679 and 5,853,801 based on discotic liquid crystalswhich have negative birefringence, is widely used.

Viewing angle compensation layers useful in the present invention areoptically anisotropic layers. The optically anisotropic, viewing anglecompensation layers may comprise positively birefringent materials ornegatively birefringent materials. The compensation layer may beoptically uniaxial or optically biaxial. The compensation layer may haveits optic axis tilted in the plane perpendicular to the layer. The tiltof the optic axis may be constant in the layer thickness direction orthe tilt of the optic axis may vary in the layer thickness direction.

Optically anisotropic, viewing angle compensation layers useful in thepresent invention may comprise the negatively birefringent, discoticliquid crystals described in U.S. Pat. Nos. 5,583,679, and 5,853,801;the positively birefringent nematic liquid crystals described in U.SPatent 6,160,597; the negatively birefringent amorphous polymersdescribed in commonly assigned U.S. Patent Application Publication2004/0021814A and U.S. patent application Ser. No. 10/745,109, filedDec. 23, 2003. These latter two patent applications describecompensation layers comprising polymers that contain non-visiblechromophore groups such as vinyl, carbonyl, amide, imide, ester,carbonate, sulfone, azo, and aromatic groups (i.e. benzene, naphthalate,biphenyl, bisphenol A) in the polymer backbone and that preferably havea glass transition temperature of greater than 180 degree C. Suchpolymers are particularly useful in the compensation layer of thepresent invention. Such polymers include polyesters, polycarbonates,polyimides, polyetherimides, and polythiophenes. Of these, particularlypreferred polymers for use in the present invention include: (1) apoly(4,4′-hexafluoroisopropylidene-bisphenol)terephthalate-co-isophthalate, (2) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalate,(3) a poly(4,4′-isopropylidene-2,2′6,6′-tetrachlorobisphenol)terephthalate-co-isophthalate, (4) apoly(4,4′-hexafluoroisopropylidene)-bisphenol-co-(2-norbornylidene)-bisphenolterephthalate, (5) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene)-bisphenol-co-(4,4′-isopropylidene-2,2′,6,6′-tetrabromo)-bisphenolterephthalate, (6) apoly(4,4′-isopropylidene-bisphenol-co-4,4′-(2-norbomylidene) bisphenol)terephthalate-co-isophthalate, (7) apoly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate, or (8) copolymers of any twoor more of the foregoing. A compensation layer comprising these polymerstypically has an out-of-plane retardation, R_(th), that is more negativethan −20 nm; preferably R_(th) is from −60 to −600 nm, and mostpreferably R_(th) is from −150 to −500 nm.

Another compensation layer suitable for the present invention includesan optically anisotropic layer comprising an exfoliated inorganic claymaterial in a polymeric binder as described in Japanese PatentApplication 11095208A.

The auxiliary layers of the invention can be applied by any of a numberof well known liquid coating techniques, such as dip coating, rodcoating, blade coating, air knife coating, gravure coating, microgravurecoating, reverse roll coating, slot coating, extrusion coating, slidecoating, curtain coating, or by vacuum deposition techniques. In thecase of liquid coating, the wet layer.is generally dried by simpleevaporation, which may be accelerated by known techniques such asconvection heating. The auxiliary layer may be applied simultaneouslywith other layers such as subbing layers and the low birefringencepolymer film. Several different auxiliary layers may be coatedsimultaneously using slide coating, for example, an antistatic layer maybe coated simultaneously with a moisture barrier layer or a moisturebarrier layer may be coated simultaneously with a viewing anglecompensation layer. Known coating and drying methods are described infurther detail in Research Disclosure 308119, Published Dec. 1989, pages1007 to 1008.

The cover sheets of the invention are suitable for use with a widevariety of LCD display modes, for example, Twisted Nematic (TN), SuperTwisted Nematic (STN), Optically Compensated Bend (OCB), In PlaneSwitching (IPS), or Vertically Aligned (VA) liquid crystal displays.These various liquid crystal display technologies have been reviewed inU.S. Pat. Nos. 5,619,352 (Koch et al.), 5,410,422 (Bos), and 4,701,028(Clerc et al.).

As should be obvious based on the preceding detailed description, a widevariety of guarded cover sheet composites having various types andarrangements of auxiliary layers may be prepared. Some of theconfigurations possible in accordance with the present invention areillustrated by the following non-limiting examples.

Guarded Cover Sheet Composite C1: TAC PVA layer carrier substrate PVAlayer TACA 60 micrometer thick polyethylene terephthalate carrier substrate iscoated on each side with a layer promoting adhesion to PVA comprisingthe polyvinyl alcohol polymer Cervol® 107 PVA (98-99% hydrolyzed,available from Celanese Corp.) that has a dried thickness of 1micrometer. Each dried PVA layer is then overcoated with a triacetylcellulose (TAC) formulation in a sequential operation. The dried TAClayers are 20 micrometers in thickness and contain 11 wt % triphenylphosphate plasticizer, 1 wt % TINUVIN® 8515 UV absorber (a mixture of2-(2′-Hydroxy-3′-tert-butyl-5′-methylphenyl)-5-chloro benzotriazole and2-(2′-Hydroxy-3′,5′-ditert-butylphenyl)-benzotriazole, available fromCiba Specialty Chemicals) and about 0.1 wt % PARSOL® 1789 UV absorber(4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane, available from RocheVitamins Inc.).

Guarded Cover Sheet Composite C2: PVA layer TAC carrier substrate TACPVA layerGuarded Cover Sheet Composite C2 is prepared in an analogous manner tocomposite C1 except that the TAC layer are applied on the carriersubstrate and the PVA layers are applied onto the dried TAC layers.

Guarded Cover Sheet Composite C3: TAC release layer primer layer carriersubstrate primer layer release layer TACGuarded Cover Sheet Composite C3 is prepared in an analogous manner tocomposite C1 except on each side of the polyethylene terephthalatecarrier substrate there is a first subbing layer that is a 0.1micrometer thick primer layer comprising poly(vinylidenechloride-co-acrylonitrile-co-acrylic acid) and a second subbing layerthat is a 0.5 micrometer thick release layer comprising polyvinylbutyral.

Guarded Cover Sheet Composite C4: abrasion resistant layer antistaticlayer TAC tie layer PVA layer carrier substrate PVA layer tie layer TACGuarded cover sheet composite C4 is prepared in an analogous manner tocomposite C1 except that 1 micrometer thick tie layers comprising a87/8.7/4.3 mixture of Carboset® 525 carboxylated acrylic polymer (NoveonInc.), Desmodur N100 isocyanate (Bayer Materials Science AG) andtrimethyl borate are coated and dried simultaneously with the TAClayers. An antistatic layer comprising Baytron® P (polyethylenedioxythiophene/polystyrene sulfonate, available from Bayer Corp) in apoly(vinylidene chloride-co-acrylonitrile-co-acrylic acid) binder isapplied onto the TAC layer on one side of the carrier substrate. Theantistatic layer contains 3 mg/m² Baytron® P and has a surfaceresistivity of about 1×10⁸ Ω/square. An abrasion resistant layer isapplied onto the antistatic layer by coating, drying and then UV curinga urethane acrylate oligomer, CN968® from Sartomer Company.

Guarded Cover Sheet Composite C5: abrasion resistant layer TAC carriersubstrate cyclic olefin polymer viewing angle compensation layerA 60 micrometer thick polyethylene terephthalate carrier substrate iscoated on one side with a triacetyl cellulose (TAC) formulation to givea dried TAC layer that is 20 micrometers in thickness and contains 11 wt% triphenyl phosphate plasticizer, 1 wt % TINUVIN® 8515 UV absorber (amixture of 2-(2′-Hydroxy-3′viewing angle compensation layer-tert-butyl-5′-methylphenyl)-5-chloro benzotriazole and2-(2′-Hydroxy-3′,5′-ditert-butylphenyl)-benzotriazole, available fromCiba Specialty Chemicals) and about 0.1 wt % PARSOL® 1789 UV absorber(4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane, available from RocheVitamins Inc.). The other side of the carrier substrate is coated with20 micrometer thick layer of a cyclic olefin polymer, Zeonor® availablefrom Nippon Zeon. An abrasion resistant layer is applied onto the TACfilm by coating, drying and then UV curing a urethane acrylate oligomer,CN968® from Sartomer Company. Finally, a viewing angle compensationlayer is applied onto the cyclic olefin polymer film. The viewing anglecompensation layer is a 3 micrometer thick layer comprisingpoly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate.

Guarded Cover Sheet Composite C6: abrasion resistant layer antistaticlayer TAC tie layer PVA layer carrier substrate PVA layer tie layer TACGuarded cover sheet composite C6 is prepared in an analogous manner tocomposite C4 except that the TAC layer on the same side of the carriersubstrate as the abrasion resistant layer is only 10 micrometers inthickness and the TAC layer on the opposite side is 20 micrometers inorder to minimize the curl of the guarded cover sheet composite.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST:

-   5 tandem coating and drying system-   8 simultaneous coating and drying system-   10 coating and drying apparatus-   12 moving substrate-   14 dryer-   14 a dryer-   14 b dryer-   15 web conveyance element-   16 coating apparatus-   16 a coating apparatus-   16 b coating apparatus-   16 c coating apparatus-   16 d coating apparatus-   18 unwinding station-   20 back-up roller-   20 a back-up roller-   20 b back-up roller-   22 coated substrate-   22 a coated substrate-   22 b coated substrate-   22 c coated substrate-   24 guarded cover sheet composite-   26 winding station-   28 coating supply vessel-   28 a coating supply vessel-   28 b coating supply vessel-   28 c coating supply vessel-   28 d coating supply vessel-   30 coating supply vessel-   30 a coating supply vessel-   30 b coating supply vessel-   32 coating supply vessel-   32 a coating supply vessel-   32 b coating supply vessel-   34 coating supply vessel-   34 a coating supply vessel-   34 b coating supply vessel-   36 pumps-   36 a pumps-   36 b pumps-   36 c pumps-   36 d pumps-   38 pumps-   38 a pumps-   38 b pumps-   40 pumps-   40 a pumps-   40 b pumps-   42 pumps-   42 a pumps-   42 b pumps-   44 conduits-   44 a conduits-   44 b conduits-   46 conduits-   46 a conduits-   46 b conduits-   48 conduits-   48 a conduits-   48 b conduits-   50 conduits-   50 a conduits-   50 b conduits-   52 discharge device-   52 a discharge device-   52 b discharge device-   54 polar charge assist device-   54 a polar charge assist device-   54 b polar charge assist device-   66 drying section-   68 drying section-   70 drying section-   72 drying section-   74 drying section-   76 drying section-   78 drying section-   80 drying section-   82 drying section-   92 front section-   94 second section-   96 third section-   98 fourth section-   100 back plate-   102 inlet-   104 metering slot-   106 pump-   108 lower most layer-   110 inlet-   112 2^(nd) metering slot-   114 pump-   116 layer-   118 inlet-   120 metering slot-   122 pump-   124 form layer inlet-   126 inlet-   128 metering slot-   130 pump-   132 layer-   134 incline slide surface-   136 coating lip-   138 2^(nd) incline slide surface-   140 3^(rd) incline slide surface-   142 4^(th) incline slide surface-   144 back land surface-   146 coating bead-   151 guarded cover sheet composite-   153 guarded cover sheet composite-   155 guarded cover sheet composite-   161 lowermost layer-   161 a lowermost layer-   161 b lowermost layer-   162 lowermost layer-   162 a lowermost layer-   162 b lowermost layer-   163 intermediate layer-   164 intermediate layer-   165 outermost layer-   165 a outermost layer-   165 b outermost layer-   166 outermost layer-   166 a outermost layer-   166 b outermost layer-   170 carrier support-   171 cover sheet-   173 cover sheet-   175 cover sheet-   177 cover sheet-   179 cover sheet-   181 cover sheet-   182 carrier substrate-   184 release layer-   200 feed line-   202 extrusion hopper-   204 pressurized tank-   206 pump-   208 metal drum-   210 drying section-   212 drying oven-   214 cast film-   216 final drying section-   218 final dried film-   220 wind-up station-   232 guarded cover sheet composite supply roll-   236 PVA-dichroic film supply roll-   238 PVA-dichroic film-   240 carrier substrate take-up roll-   242 opposing pinch roll-   244 opposing pinch roll-   250 polarizer plate-   252 polarizer plate-   254 polarizer plate-   260 LCD cell-   261 layer promoting adhesion to PVA-   262 low birefringence polymer film-   264 moisture barrier layer-   266 antistatic layer-   268 antiglare layer-   272 viewing angle compensation layer

1. A guarded cover sheet composite comprising a temporary carriersubstrate having a first cover sheet comprising a first lowbirefringence film on one side and a second cover sheet comprising asecond low birefringence film on the other side.
 2. The composite ofclaim 1 wherein said carrier substrate comprises polyethyleneterephthalate.
 3. The composite of claim 1 wherein said first and/orsecond low birefringence polymer films comprise cellulose esters.
 4. Thecomposite of claim 1 wherein said first and/or second low birefringencepolymer films comprise triacetyl cellulose, polycarbonate, or cyclicolefin copolymers
 5. The composite of claim 1 wherein said first and/orsecond low birefringence polymer films comprise triacetyl cellulose. 6.The composite of claim 1 wherein said first and second low birefringencepolymer films independently have a thickness of 5 to 50 micrometers. 7.The composite of claim 1 wherein said first and second low birefringencepolymer films comprise the same material.
 8. The composite of claim 1wherein said first and second low birefringence polymer films comprisedifferent materials.
 9. The composite of claim 1 wherein said carriersubstrate has a thickness of 10 to 125 micrometers.
 10. The composite ofclaim 1 wherein said carrier substrate has a thickness of 10 to 50micrometers.
 11. The composite of claim 1 wherein at least one coversheet further comprises a layer promoting adhesion to polyvinyl alcohol.12. The composite of claim 1 wherein both cover sheets further comprisea layer promoting adhesion to polyvinyl alcohol.
 13. The composite ofclaim 12 wherein said layer promoting adhesion to polyvinyl alcohol isadjacent to said carrier substrate.
 14. The composite of claim 12wherein said layer promoting adhesion to polyvinyl alcohol is on theside of the cover sheet opposite to the carrier substrate.
 15. Thecomposite of claim 12 wherein said layer promoting adhesion to polyvinylalcohol comprises hydrophilic polymer.
 16. The composite of claim 12wherein said layer promoting adhesion to polyvinyl alcohol comprisespolyvinyl alcohol.
 17. The composite of claim 12 wherein said layerpromoting adhesion to polyvinyl alcohol comprises pressure sensitiveadhesive.
 18. The composite of claim 12 wherein said layer promotingadhesion to polyvinyl alcohol has a thickness of between 0.1 and 5micrometers.
 19. The composite of claim 1 wherein at least one coversheet further comprises an auxiliary layer.
 20. The composite of claim 1wherein both cover sheets further comprise an auxiliary layer.
 21. Thecomposite of claim 19 wherein said auxiliary layer is an abrasionresistant layer, a low reflection layer, an antireflection layer, anantiglare layer, a compensation layer, an antistatic layer, a tie layeror a barrier layer.
 22. The composite of claim 21 wherein said auxiliarylayer is an abrasion resistant layer.
 23. The composite of claim 22wherein said abrasion resistant layer comprises a radiation curedacrylic polymer.
 24. The composite of claim 1 wherein the carriersubstrate further comprises a release layer on one or both sides. 25.The composite of claim 21 wherein said auxiliary layer is a compensationlayer comprising (1) a poly(4,4′-hexafluoroisopropylidene-bisphenol)terephthalate-co-isophthalate, (2) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalate,(3) a poly(4,4′-isopropylidene-2,2′6,6′-tetrachlorobisphenol)terephthalate-co-isophthalate, (4) apoly(4,4′-hexafluoroisopropylidene)-bisphenol-co-(2-norbornylidene)-bisphenolterephthalate, (5) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene)-bisphenol-co-(4,4′-isopropylidene-2,2′,6,6′-tetrabromo)-bisphenolterephthalate, (6) apoly(4,4′-isopropylidene-bisphenol-co-4,4′-(2-norbornylidene) bisphenol)terephthalate-co-isophthalate, (7) apoly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate, or (8) copolymers of any twoor more of the foregoing.
 26. The guarded cover sheet composite of claim21 wherein said auxiliary layer is a compensation layer that has anout-of-plane retardation that is more negative than −20 nm.
 27. Theguarded cover sheet composite of claim 1 wherein said first and secondcover sheets have a moisture vapor transmission rate of less than 500grams per meter squared per day.
 28. A method of forming a polarizingplate comprising providing a guarded cover sheet composite comprising atemporary carrier substrate having a first low birefringence cover sheeton one side and a second low birefringence cover sheet on the otherside, peeling the first and second cover sheets from the carriersubstrate; providing a dichroic film; and simultaneously bringing saidfirst and second cover sheets into adhesive contact with the dichroicfilm on opposite sides of said dichroic film.
 29. The method of claim 28wherein the cover sheets further comprise a layer promoting adhesion topolyvinyl alcohol, and simultaneously bringing said cover sheets intocontact with said dichroic film such that the layer promoting adhesionto polyvinyl alcohol in each of said two cover sheets is in contact withsaid dichroic film.
 30. The method of claim 29 wherein said layerpromoting adhesion to polyvinyl alcohol is adjacent to said carriersubstrate.
 31. The method of claim 29 wherein said layer promotingadhesion to polyvinyl alcohol comprises polyvinyl alcohol or a pressuresensitive adhesive.
 32. The method of claim 28 wherein glue is appliedsimultaneously with bringing said dichroic film and said cover sheetsinto contact.
 33. The method of claim 32 wherein said glue comprises apolyvinyl alcohol solution.
 34. The method of claim 28 wherein aftersaid dichroic film and cover sheets are brought into contact thepolarizing plate is dried.
 35. A method of making a guarded cover sheetcomposite comprising providing a temporary carrier substrate andapplying a first coating which will form a first low birefringence filmto one side of said carrier substrate; and a second coating which willform a second low birefringence film to the opposite side of saidcarrier substrate, and drying said coatings.
 36. The method of claim 35wherein said coatings are applied and dried sequentially.
 37. The methodof claim 35 wherein said coatings are applied and dried simultaneously.38. The method of claim 35 wherein the coatings are applied using amulti-layer slide bead coating method, a multi-layer curtain coatingmethod or a multi-manifold extrusion die coating method.
 39. The methodof claim 35 wherein one or more additional auxiliary layers are coatedon the carrier substrate prior to coating the low birefringence films.40. The method of claim 35 wherein one or more additional auxiliarylayers are coated on the low birefringence films.
 41. The method ofclaim 35 wherein the one or more additional auxiliary layers are coatedwith the low birefringence films using a multi-layer coating method. 42.The method of claim 35 wherein the one or more additional auxiliarylayers are coated on the low birefringence films after the lowbirefringence films are dry.
 43. The method of claim 35 wherein anadhesive layer is coated on the carrier substrate prior to coating thelow birefringence films.